I cancelled a domain controller demotion. Why can't I demote the domain controller now?

A. Once you start a domain controller (DC) demotion, you don't have an option to cancel it; however, you might have stopped the demotion via the Task Manager or by getting the process ID with tlist, then using the kill command. Neither approach is supported, and if you try to run DCPROMO again, you might get the following error message (depending on the stage the DCPROMO process was in when you stopped it):

Active Directory is already being installed or removed on this computer. That operation must finish before another may be attempted. 

The only way to remove the DC information is to manually remove it using NTDSUTIL. See How do I remove a nonexistent domain controller? for details.