This is very frustrating and as you can probably tell by now there is no ONE fix to this. And the fact is that there are MANY reasons why this may be failing, so the key thing is to try and figure our WHAT is causing this, and only then may you be able to apply an appropriate fix.
As it was mentioned earlier in this thread the 1st thing you should try is to run the following command in Terminal.
Step 1: Enter this in Terminal:
log stream --predicate 'category == "AutoUnlock" AND eventMessage contains "fail with error"'
Leave terminal running and
Step 2: Attempt an Auto-Unlock
Then check to see what error was returned in Terminal. In my case it appeared that I was getting a timeout error, specifically "ranging timeout error" which usually indicates a Wi-Fi issue, so I simply went into System Settings/Network/Wi-Fi/Details/TCP/IP and clicked on "Renew DHCP Lease" and my problem was fixed immediately without having to jump through any of the previous loops of turning things on and off, restarting, etc.
****** So add "Renew DHCP Lease" to the list of things to try.
If you get some other error (and I am sure you will) try and research and see what that error is related to, and you may be able to find the root of your issue.