Your Mac was unable to communicate with your Apple Watch.

This isn’t my solution but was posted in another thread by stoska992, but it worked for me!

I've finally fixed it! I found a bunch of errors related to "AutoUnlock" in the Console that was hinting to me that there was some invalid state on my Mac with keys and plists not being reset properly. 

Steps (follow at your own discretion)

1. Open "Keychain Access"
2. In "View", enable "Show Invisible Items"
3. Search for "Auto Unlock"
4. You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
5. Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
6. Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
7. There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
8. Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
9. Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock"
10. There should be two files "ltk.plist" and "pairing-records.plist"
11. Delete both files
12. Open "System Preferences" and try enabling auto unlock. You may need to enable it twice, the first attempt will fail.

Basically below are the steps:

Steps (follow at your own discretion)

1. Open "Keychain Access"
2. In "View", enable "Show Invisible Items"
3. Search for "Auto Unlock"
4. You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
5. Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
6. Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
7. There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
8. Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
9. Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock"
10. There should be two files "ltk.plist" and "pairing-records.plist"
11. Delete both files
12. Open "System Preferences" and try enabling auto unlock. You may need to enable it twice, the first attempt will fail.
13. RESTART MAC and try

This isn’t my solution but was posted in another thread by stoska992, but it worked for me!

I've finally fixed it! I found a bunch of errors related to "AutoUnlock" in the Console that was hinting to me that there was some invalid state on my Mac with keys and plists not being reset properly. 

Steps (follow at your own discretion)

1. Open "Keychain Access"
2. In "View", enable "Show Invisible Items"
3. Search for "Auto Unlock"
4. You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
5. Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
6. Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
7. There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
8. Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
9. Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock"
10. There should be two files "ltk.plist" and "pairing-records.plist"
11. Delete both files
12. Open "System Preferences" and try enabling auto unlock. You may need to enable it twice, the first attempt will fail.

Yay! This worked (after trying all the other stuff that did not work!). Why are we so obsessed with making little things like this work?! I mean, typing my login password takes about 1/2 a second! I spent 3 hours troubleshooting this! Whats wrong with us???

Steps (follow at your own discretion)

1. Open "Keychain Access"
2. In "View", enable "Show Invisible Items"
3. Search for "Auto Unlock"
4. You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
5. Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
6. Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
7. There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
8. Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
9. Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock"
10. There should be two files "ltk.plist" and "pairing-records.plist"
11. Delete both files
12. Open "System Preferences" and try enabling auto unlock. You may need to enable it twice, the first attempt will fail.

I had the same issue with Apple Watch 6 (watchOS 7.1) and MacBook Pro 13 early 2015 (Big Sur). So I repaired it with some steps:

1) I disabled Keychain synchronisation on MacBook (Settings/iCloud/Keychain)

2) I made steps in this thread https://discussions.apple.com/thread/251803595?answerId=253458029022#253458029022

• • Open "Keychain Access"
  • In "View", enable "Show Invisible Items"
  • Search for "Auto Unlock"
  • You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
  • Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
  • Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
  • There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
  • Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
  • Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock"
  • There should be two files "ltk.plist" and "pairing-records.plist"
  • Delete both files

3) Disconnected Watch from iPhone

4) Connected back Watch to iPhone (set it from backup)

5) Restarted MacBook

6) Turned on Keychain in iCloud settings

7) Enabled Auto Unlock for Watch in System Prefs

So I followed this thread and I've fixed my problem. Hopefully this'll help some of you.

For reference I am currently using iPhone 11 with iOS 14 and Apple Watch Series 5 with Watch OS 7. This issue was appearing in Catalina but also occurred when I started beta testing Big Sur, therefore I assume this solution will work for both MacOS versions.

Firstly I followed the KeyChain reset steps above (thank you Mark_T)

Steps (follow at your own discretion)

1. 1. Open "Keychain Access"
   2. In "View", enable "Show Invisible Items"
   3. Search for "Auto Unlock"
   4. You should see a whole bunch of application passwords for "Auto Unlock: XXXX's ..."
   5. Select all records and delete (this will reset/disable auto unlock on other Macs if you use multiple Macs)
   6. Whilst still in "Keychain Access", search for "AutoUnlock" (no space)
   7. There should be 4 entries for "tlk" "tlk-nonsync" "classA" "classC"
   8. Select 4 records and delete (don't worry if they re-appear, the system repairs this automatically)
   9. Open "Finder" and navigate to "~/Library/Sharing/AutoUnlock" (Copy and paste this in Go To Folder without quote marks, to find the files)
   10. There should be two files "ltk.plist" and "pairing-records.plist"
   11. Delete both files
   12. Open "System Preferences" and try enabling auto unlock. You may need to enable it twice, the first attempt will fail.

This didn't work on its own for me so I followed the advice of others as well, as follows

Go to System preferences and click in to icloud and disable keychain

reboot the mac

don't enable keychain yet

Got to System Preferences, Security and Privacy and tick the box to enable unlock with Apple Watch

This will produce the error (I think it resets something because Keychain is not enabled)

Go back to icloud settings and enable Keychain, wait for it to enable

Back in Security and Privacy, tick the box to enable unlock with Apple Watch

Finally solved my problem. Thanks to all for contributing.

mark__t was on it! Good Job! I did have to do a bit more to get my brand new MacBook and new MacMininto work. I followed your directions but it didn't work until I disabled Keychain in iCloud settings on both computers, re-booted each of them, then re-enabled Keychain in iCloud.

I think you hit the nail on the head! I found Auto Login from several devices from YEARS ago in my Keychain. When I followed mark__t's advice, there were like 30 of them in my Keychain. Cleaning them out, rebooting without iCloud Keychain, and finally re-enabling iCloud Keychain allowed it to work the way it should. Thanks mark__t.... You're awesome!

Had the same problem plus the handoff function didn't work after upgrade to Big Sur. After contacting support the problem has been solved after the first step they recommended to find out if the problem is related to the MacBook or iCloud account. Here is what I did step by step, I suppose some steps are not needed, I wt trying different things:

• Support asked me to check if the problem appears if I create a new user account in MacOS. So I created a new test user in MacOS (in my case it wasn't even an admin user, so restart was not required) with minimum required configuration (I skipped all steps that were not necessary like touchID config.)
• log in to this new user account, setup iCloud for this new account
• turn on "unlock with Apple Watch" function settings
• turned out that both handoff function and this one work properly for this new account - so I know the problem is not related to the hardware
• log in back to the original MacOS account - here I checked iCloud settings, logged out from the iCloud (worked after few attempts due to pending picture synchronisation)
• restart
• enabling "Use your Apple Watch to unlock apps and your Mac." - it worked after 2nd or maybe 3rd attempt, I'm not sure but definitely the 1st attempt resulted in the same error message the story has started from
• fixed! :) And also handoff works properly with my watch and iPhone

Hope it can help others to fix the issue!

"Unable to connect to watch" problem arose when I transitioned from iPhone 11 to 12 pro max. Using series 5 watch, new iMac. Same problem with watch unable to open my MacBook. I tried everything I could find in the discussion groups: deleting keychain access files and related, unpairing and pairing watch as new, factory reset on watch, etc, etc, etc. Over and over.

This finally worked for me. On both the iMac and MacBook, deleted the iCloud account. Then re-added iCloud on each. (I had local contact files, etc, kept on the machines.) When then trying to check the "Use your Apple Watch ..." box in Security & Privacy, did bump into a message that my Apple Account settings needed to be updated/verified. Took care of that. And, low and behold, can check that pesky "Use your Apple Watch ..." box on both the iMac and the MacBook. And, watch does do its thing when I need to log into either.

@hakarlmedskyr I had the same error message - "Your Mac was unable to communicate with your Apple Watch."

I initially got the message that my watch needed to be signed in to my iCloud account - which it definitely was.

So, I also followed other suggestions to try to get the watch to unlock my Mac:

• Signed out of iCloud on my Mac
• Shut down
• Restarted my Mac (M1 MacBook Air, Big Sur v11.2.3)
• Went to System Preferences -> Security & Privacy -> Use your Apple Watch to unlock apps and your Mac

At least I didn't get the "Your watch must be signed into your iCloud message"…

However, I got the "Your Mac was unable to communicate with your Apple Watch." message. Here are the next steps I took:

• I made sure my watch (Series 4) was on the same WiFi network as my iPhone 11 and Mac
• Made a phone call from my watch
• I left a voicemail at the number I called from my watch (just wanted to make sure that my watch was actually connected to my phone, and also that the watch actually making the call)
• I went back to System Preferences -> Security & Privacy -> Use you Apple Watch to unlock apps and your Mac and checked the box (to unlock apps and Mac)

Then it worked!

I hope that helps someone.

I was able to resolve this problem on my new iMac running Big Sur by powering off my Apple Watch, shutting down my iMac, and then powering on my Apple Watch, turning on my iMac, and finally checking the Use your Apple Watch to unlock apps and your Mac check box. That was several days ago, and my Apple Watch is still able to unlock my iMac when it wakes up.

I got the same problem for months.

The approach with "Keychain Access" doesn't work for my case as I couldn't find the "View" and "Show Invisible Items" at all. I even tried to reset my Apple watch (series 3), as well as upgrade the OS on the watch (7.3.2) and my MacBook (Big Sur), but it still doesn't work either.

After a few search on the Internet, I just found another solution that works for me, so you can give it a try. Here it is:

1. On your iPhone, go to the Watch app
2. Go to the passcode section
3. Switch off the "Unlock with iPhone" toggle
4. On your Macbook, go to System Preferences > Security & Privacy
5. Tick on "Use your Apple Watch to unlock your apps and your Mac" (and it now should be working fine)
6. Back to iPhone and switch on the "Unlock with iPhone" toggle (optional, if you want to keep the "unlock Apple Watch by iPhone" feature)

My hypothesis is that Apple doesn't want users to use iPhone to unlock MacBooks (I don't know why).

I did everything in here and nothing worked so I decided to sign out of iCloud on both the phone and my Mac then signed back in. Apparently there was an iCloud Keychain issue on my phone which doing this fixed it. Once I signed back in, I tried again to tick the unlock my Mac with my Watch and it worked.

Here is my variation on this thread.

After using the suggestion on here - https://discussions.apple.com/thread/251803595, I did the following:

1. Toggle "KeyChain" off
2. Shutdown my iMac
3. Power Up the iMac
4. Toggle "KeyChain" on, wait until it is checked
5. Your AppleWatch will prompt you to log into your iCloud account. If you have it saved, you can scroll through it and select it. Otherwise, you will need to type it into the tiny AppleWatch window
6. Go to System Preferences and enable the "Use Your Apple Watch..." and enter in your Mac login password. it should work. It may take a while for the two devices to communicate

After 2-3 hour debugging, I fixed by "Reset default Keychains" from Keychain Access app > Preference, logout your user and make sure to enable Keychain again from Mac iCloud