I tried all the procedures described at the following link, no success. The problems started suddenly, on macOS Catalina, and Xcode 12.01. I've updated to macOS bid sure and Xcode 12.2, the problem still persists, and I can't publish on TestFlight.
What to do?
https://developer.apple.com/forums/thread/127212
What to do?
https://developer.apple.com/forums/thread/127212