After several weeks of the watch unlocking my Mac, it suddenly stopped working. So, restarted MacBook Pro (M1 Max, Ventura 13.4.1), iPhone 13 (iOS 16.5.1), AND Watch 5 (WatchOS 9.5.2). Doesn't help. This is a VERY OLD BUG!!
Why does the MacOS continue to assert "Watch must be signed into iCloud to allow it to unlock your Mac. You can sign in using the Apple Watch app on your iPhone." That's a bug in itself. There IS NO iCLOUD sign in within the Watch app. The Watch and the phone ARE communicating with each other AND Find Devices on the Watch does find the MacBook Pro!
At least once, I've had to resort to completely reinstalling the Watch (time consuming!!) in order to fix this. Does anyone have a better idea? It seems like the MacOS coding team isn't paying attention (per the misleading and wrong response in System Preferences).