The reason for the 'Internal Server Error: 400' is that the credentials of the user attempting to enroll in the Mobile Security for Enterprise application do not exist in the Mobile Inventory of Vision One.

To resolve this error, simply add the user to the Mobile Inventory by following these steps:

1. Log in to the Vision One console.
2. Go to Mobile Security Operations > Mobile Inventory > Users tab.
3. Click on "Add Assignments" or "Invite Users" (The button label may vary based on your organization's MDM).
4. Assign the users to be added to the Vision One Mobile Inventory.
5. Finally, attempt to re-enroll the user in the Mobile Security for Enterprise Application.

By following the steps above, the user should be able to enroll successfully in Mobile Security.