I just had this issue on Xcode 12.1 upgrade. Very annoying. It might have something to do with me just having 1 account on my mac mini where I host the build machine, and that user is logged in automatically when mini starts.
Anyway, I fixed by:
creating a new account on the mac mini - called xcodebot
open xcode, go to Xcode Server settings and turn off and on, selecting new xcodebot user
It then told me xcodebot wasn't logged in, so i tapped login which took me to setting up account on mac
Once logged in, my dev machine started building my integrations
👍