Okay, well I've discovered the client has created the developer account as an individual account not as an organisation - that explains why I can see App Store Connect but not the team in developer.apple.com Why the main owner/admin account won't work in Xcode (even on a brand new M1 Mac) is still a mystery but I am gonna get my client to migrate their individual account to an organisation account and add me as an app manager or developer and then it should all work.

I was getting this on an enterprise app built with Xcode 12.4 on Catalina. I rebuilt with Xcode 12.4 on an M1 Mac with Big Sur and the app now opens fine on iOS15. I used automatic signing on both - so it wasn't an issue with manual signing for me. I was and still am targeting iOS13 btw - the Xcode 12.4 on Big Sur doesn't have the iOS15 options that come with Xcode 13.