Cannot send iMessages on my Mac but can receive them

THANK YOU for taking the time to post your response here. Deleting com.apple.facetime: registrationV1 from Keychain Access worked!

I have been trying everything to get iMessage back on my MacBook after getting a new phone/phone number and having to reset my AppleID.

This worked immediately.

So thankful :)

I did this, tried to mark it helpful, and was not able to. It did work for me, perfectly. Happily, I had copied the instruction onto a sticky note, because today, several months later, I again was unable to send messages from my laptop. I went into keychain access as directed, and found that the com.apple.facetime had come back today, at 11:00 a.m. I followed the rest of the instructions, and it worked like a charm, again.

Praise Jeebus!!! This just worked for me after a torturous amount of frustration trying all the myriad of ways I'd read to fix this issue. Thank you, thank you!!! (And in a hysterical turn of events - this page just deigned to give me the dreaded red exclamation point saying my comment could not be posted. Oh, irony, you are not lost on me.)

THANK YOU @cabrownfromottawa for the following advice that was magical!

Close all applications, specifically Messages

Reboot

Spotlight search for Keychain Access

Search in Keychain Access for - com.apple.facetime

Right click on and delete - com.apple.facetime: registrationV1

Reboot

Launch Messages

What happens when you try to send a message?

Please take each of the following steps that you haven't already tried. Stop when the problem is resolved.

1. Open the Messages preferences window and select the Accounts tab. On the left is a list of the messaging accounts that you've defined. Select iMessage. Make sure that the Apple ID shown is valid. If it is, check the box marked

Enable this account

if it's not already checked.

2. Sign out of iMessage on all your devices and sign back in. If you've enabled two-step verification for the Apple ID that you use with iMessage, you must generate an application-specific password and enter it when prompted. Test.

Note: iMessage and iCloud are not the same, even if you use the same Apple ID for both. You must sign out of iMessage, not iCloud.

3. Take this step if you have an iPhone linked to the same Apple ID as the Mac.

Follow the instructions in this support article, under the heading

Set up SMS and MMS with Continuity

If text forwarding is already enabled on the phone, disable it and then re-enable it.

Quit and relaunch Messages on the Mac.

Credit for this observation to ASC member Apple Advocate.

4. There may be a problem with the Apple ID account.

Click the Support link at the top of this page to schedule a call from Apple Support. You might not be charged for the call if you select "Apple ID"—not the hardware model—as the product you're asking about.

What happened when you took Step 4?

Only Apple Support can solve the problem. You'll have to be persistent. I'm afraid I can't do anything about poor customer service as I have nothing to do with Apple.

Might be a corrupt .plist.

Do a backup, preferably 2 backups on 2 separate drives.

Quit the application.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then go to Preferences/com.apple.iChat.plist. Move the .plist to your desktop.

Restart the computer, open the application and test. If it works okay, delete the plist from the desktop.

If the application is the same, return the .plist to where you got it from, overwriting the newer one.

If that doesn’t work, repeat the above but delete the com.apple.imagent.plist

Thanks to leonie and Ralph Johns (UK)for some information contained in this.