We had Apple Dev Support look into the issue and here is what they replied with. Hopefully this will be of some help to others that are having the same problem and for the same reason: This error occurred because a different Apple ID than the one you invited in App Store Connect accepted the tester invitation for [redacted email address]. A tester can only link to one Apple ID. To unlink the two Apple IDs, take the following steps in App Store Connect: Remove the affected tester on the “All Testers” pane in TestFlight for all apps. Note: Testers in the “Deleted” state aren't fully removed from TestFlight until their last installed build expires. Testers have access to installed builds for up to 90 days, even if you manually expire the build in App Store Connect. Invite the tester again. Make sure that the tester is signed in to the TestFlight app with the correct Apple ID when they accept the new invitation.