Apparently developers should save Keychain info in addition to using Backup to setup a new Mac. I did not! So I created new Developer and Distribution certificates and they show in Keychain Access. I use xCode automatic signing but I still get the ".. could not be found in the keychain" error. Suggestions to resolve this would be much appreciated!

I had this same problem June 28 2021 with xCode 12.5. I like to add a short descriptive comment at the start of a func() { But apparently it causes errors if the comment is not on a new line. The resulting error messages are very confusing. It cost me a few hours of misguided "fixes" and restoring from backups, before finding this forum post and explanation. Thanks for posting! Apple is overdue to fix this.

Patrick: thanks for posting your reply about "Removed from Sale" status and TestFlight. I updated an old app and successfully uploaded it for TestFlight testing. However, testers get the error message that it is not available or doesn't exist when trying to install it. Apparently it's because the old app is currently listed as "Removed from Sale". It was removed because it was developed using Flash based tools that don't meet App Store requirements. A similar update to another old app worked fine with TestFlight but it was still "Ready for Sale". My new version of the app using xCode 12.5 and SwiftUI tests OK when locally installed. So it appears that I have to submit it for App Store approval before it can be tested using TestFlight!!