Since upgraded to BigSur my mac will not unlock with apple watch.

Update to watchOS 7.1, as it has a fix for unlocking Macs

If that does not work, it should, but if it does not work, then read the following threads

Check out this thread on fixing Apple Watch unlocking Mac

https://discussions.apple.com/thread/251803595?answerId=253458029022#253458029022

Then follow with this thread

https://discussions.apple.com/thread/251826134?answerId=253504359022#253504359022

And

I also found that it took awhile for the iCloud Keychain records to propagate and merge on all the systems. More so when you have multiple devices sharing the keychain (I have 4 Macs, the iPhone, and the Apple Watch). I followed both of the above threads, but it was not reliably working until the next day, when I tried again. And I did have to click the enable check box twice the next day, but then it stuck.

And this thread says they also had to toggle the Apple Watch -> Settings -> Siri -> Listen for "Hey Siri" option off then on

https://discussions.apple.com/thread/251803595?answerId=253782332022#253782332022

Hints (you will understand when you read the first and second threads):

Applications -> Utilities -> Keychain Access

Finder -> Go menu -> ~/Library/Sharing/AutoUnlock

System Preferences -> iCloud -> Keychain

What do you mean “changed the settings”?

My last conversation with Apple they wanted me to create a whole new log in profile. We tested that and it worked. Do they recommendation was move everything over to the new profile but that don’t make sense to me. Something is “stuck” in the original profile and if we could isolate what that is I’m sure it would be a global fix for all.

I just went to Preferences and typed in "watch", which took me to Security & Privacy", where I unlocked, checked the appropriate box, and re-locked. I'm guessing you tried that and it didn't stick.

Big Sur Update - again, kicks me out of Apple Watch auto unlock. Had to repeat the log out of iCould and re-log back in again. That is the only fix I could fine (thanks again above for the tip - Apple agents apparently aren't aware of that fix). I'm wondering if this is going to keep occurring every time there is an update? Seems to be that way so far for me.

Same here, after the Big Sur update I cannot get my watch to unlock my MacBook Pro. I've tried every fix I could find and nothing has worked.

I've tried every suggestion above. The apple support says "bug will be fixed in next release." Hmmm....seems to be a common problem that is with every release, and only affects some users.

I had the same thing happen, but changing the settings again did it. It's a bit less regular about it than it used to be though.

Correct :/

You sir, are a genius! That worked for me also. Thank you!

So the fix for me was also to sign out of iCloud and back in again on the Mac. Fixed it immediately, thanks for finding a fix!!!!