An app is available for users and has an MDM policy enabled. An end user clicks on the install pre paid option and is then taken to the VPP enrollment page. After enrollment is complete, the end user goes back to the App Center App. However, the App Center App shows the app is already getting installed on the device, so the end user doesn't click on Install (since it appears to already be installing).
Subsequently, when the user goes back into the App Center App and attempts to re-install the app, he gets the following error message:
showAlertView Cannot Update App: The previous version of the app was installed without MDM and so it cannot be updated via MDM. Please remove the previous version from the device and install the new version again.
There is a setting on iOS devices that allows app auto-download. If this setting is enabled (which it is typically is by default), then it may interfere with App Center's ability to install an app via MDM. This issue occurs because the iOS auto-download occurs before App Center can perform the MDM installation.
Have the end user uninstall the app and then disable the automatic app downloads setting in iOS. Then reinstall the app via App Center MDM.