use watch to open mac

I doubt it.

Is Security & Privacy reporting your Mac is "unable to communicate with Apple Watch" even though it is on your wrist, unlocked, you are signed in to iCloud, and otherwise meet all the requirements you already know about and have been following for several years?

The dialog is misleading and unhelpful. When I experienced that problem I found I had to open the Messages app on the Mac (of all things), and "sign in". Once I did that, I was able to select "Allow your Apple Watch to unlock your Mac".

Also, shortly thereafter, a Notification appeared that Wi-Fi calling was available.

It is certainly possible that signing in to Messages was unrelated and unlocking the Mac just spontaneously happened to start working at that time, but nothing else was helping. Signing out / in, rebooting all the devices, etc didn't work but signing in to Messages did.

As it turned out, mine stopped working after updating to WatchOS 6.0.1 😖

Great! Thanks for the update.

My “unlock with Apple Watch” option reappeared after restarting the Mac. The feature certainly seems to be one of the Watch's most fickle idiosyncrasies.