Universal Clipboard not working.

2. Use Terminal Commands

If Universal Clipboard is still not working on your Mac you can attempt to fix it with Terminal commands. However, be extremely cautious with executing code in the Terminal app. A wrong move could brick your Mac.

• Open the Terminal app, using Finder.
• Execute the command available below to check if ClipboardSharingEnabled is equal to zero. If it is, proceed to the next step:
• Command: defaults read ~/Library/Preferences com.apple.coreservices.useractivityd.plist
• Copy/Paste the command string available below and press ‘Enter’ to delete ClipboardSharingEnabled = 0 (only if it’s ‘0’):
• Command: defaults delete ~/Library/Preferences/com.apple.coreservices.useractivityd.plist ClipboardSharingEnabled
• Reboot your Mac and test out the Universal Clipboard. Does it work now?
• Extra: You can also execute the following command to set the default ClipboardSharingEnabled setting on your Mac:
• defaults write ~/Library/Preferences/com.apple.coreservices.useractivityd.plist ClipboardSharingEnabled 1

This worked for me too

I tried

defaults read ~/Library/Preferences com.apple.coreservices.useractivityd.plist

and it says

2020-12-24 01:07:09.499 defaults[2816:45183] 
The domain/default pair of (/Users/mhutson/Library/Preferences, com.apple.coreservices.useractivityd.plist) does not exist

What now? Thanks.

And then stopped working after like 10 minutes. FML. The file stops updating for some reason.

I used the terminal command above and set the default value of ClipboardSharingEnabled to 1 . It has been been stable for me too now. Thanks!