Could not Complete AirTag Setup - Sign in with your Apple ID in Settings

This is guidance rather than a question. There's a 2021 thread on this but it's now locked. One solution given was turn off Share My Location in settings, then turn it on again. This did not work for me. What did work was turning Share My Location off, rebooting the phone, then turning it on again. If you do this don't forget to re-enable Send Last Location.

Then I used the Find My app (with the map), Device, + to add the AirTag. Another oddity is that while investigating all this, I was prompted in the Find My app to sign in - but was already signed in. It seems there were some Ts & Cs that needed acknowledging, once done, FInd My app was working again.