I had the hardest time trying to get this one computer added. It kept telling me it was already added.
First, the post that helped:
@RandSol , For our issue, we can try to delete all GUIDs under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Enrollments to see if it can be enrolled.
Note: Make sure to NOT delete Context, Ownership, Status and ValidNodePaths.
The following link for the reference:
Note: Non-Microsoft link, just for the reference.
Here was the error code that started it: Error 8018000a: The device is already enrolled.
Some of the registry entries would not delete but it did fix the issue.