An app update errors with the following output: Error: The user is already being prompted

This error typically means the users are being prompted to close an app to allow an update, but they are ignoring the prompt and keeping the app open which prevents the update. Apple requires that the app, needing the update, be closed prior to the update to initialize.

For devices in Single App Mode, they will need to be taken out of Single App Mode or restarted for the App Update to initialize.