I have the same issue . The IAP works with iPhone iOS 14, but some other iPhone with iOS 13 system have the error. Tried some others method to check the issue ,but not work!!! I found a lot of developers have the same issue, it may be the bug in iOS IAP sandbox environment.About some days before, IAP works fine.

同样的问题： Errors: ( "Error Domain=ITunesTransporterErrorDomain Code=1129 \"An error occurred saving your changes to the Apple database. This problem may be a transient issue on the Apple side. If the problem persists for more than an hour, please contact your iTunes representative. (1129)\" UserInfo={NSLocalizedRecoverySuggestion=An error occurred saving your changes to the Apple database. This problem may be a transient issue on the Apple side. If the problem persists for more than an hour, please contact your iTunes representative. (1129), NSLocalizedDescription=An error occurred saving your changes to the Apple database. This problem may be a transient issue on the Apple side. If the problem persists for more than an hour, please contact your iTunes representative. (1129), NSLocalizedFailureReason=An error occurred saving your changes to the Apple database. This problem may be a transient issue on the Apple side. If the problem persists for more than an hour, please contact your iTunes representative. (1129)}" )