“The following error occurred attempting to join the domain “frankfu.com”:
The request is not supported.”
A: This server was once a domain controller. During demotion, something prevented the removal of the following registry value name:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Parameters\
DSA Database file
Delete that “Dsa Database File” value name and attempt to join the domain again. It should work this time. If you take a gander at the %systemroot%\debug\netsetup.log, you’ll see another clue that this is your issue:
NetpIsTargetImageADC: Determined this is a DC image as RegQueryValueExW loaded Services\NTDS\Parameters\DSA Database file: 0x0
NetpInitiateOfflineJoin: The image at C:\Windows\system32\config\SYSTEM is a DC: 0x32