I have the same error! The app was OK on testFlight. It was even approved for the external Test Flight group. Once we submitted the app to review on AppStore Connect, we got this email. I build & upload using the Xcode, and it is not the first version of the app. Any suggestions?

Thanks all for your help. @sysedit : Do you know if it does matter how clients get the app? For example, if they downloaded the app using an Apple Business Manager that is connected with MDM server, OR by uploading package file and the signing certificate into the MDM server?  I'm not sure if this way (send plist to the app) only works when the MDM sign the package ?

@sysedit Thank you so much. those links are really so helpful