To troubleshoot why you are not able to add or remove a domain controller from an Active Directory domain, here are some suggested things to do:
- Check if the computer account for the new Domain Controller has been created in the domain.
- Verify the Domain Controller is running the latest updated versions of the software.
- Check that the correct Domain and Forest Functional Levels are selected.
- Check for any mis-configurations in the Active Directory replication and make sure it is in sync across all Domain Controllers.
- Verify the domain controller is able to communicate on all the correct ports.