I am facing the same issue, where I am unable to invite any other device for test the TestFlight app. I have followed the Engineering has provided the following information regarding this issue: This error occurred because a different Apple ID accepted the tester invitation for ***@.com, and a tester can be linked to only one Apple ID. To unlink the two Apple IDs, take the following steps in App Store Connect: Remove the impacted tester from the All Testers tab in TestFlight for all apps. Note: Testers in the Deleted state aren't fully removed from TestFlight until their last installed build expires. Re-invite the tester. Make sure that the tester is logged in to the TestFlight app with the correct Apple ID when they accept the new invitation. But No luck