I ran into this as a freelance developer after asking my client to add my Apple ID to his individual App Store account. I then used my Apple ID to log into Xcode, and when I had to select a team, I selected the "personal team" option. I didn't know what that meant, but it was the only option available. What I should have done instead was log into Xcode with his Apple ID, which is counterintuitive because the rise of 2-factor authentication trains us not to use someone else's login info. But apparently that is the only way for someone besides the account owner to add an app to an individual account.
(I could log into his App Store Connect account with my own Apple ID, but couldn't add an app that didn't already have an app ID created. And I couldn't log into his Provisioning Portal with my Apple ID to manually create the app ID, even though he gave me the maximum available permissions in his account.)
Anyway, I tried Dipo's trick a few times without success, then tried ldittel's suggestion, but via email instead of phone. The first Apple representative told me that deleting the bundle ID wasn't possible, then I wrote back and said I think it is and linked to this thread and then a second representative deleted it for me.