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:

1. Check if the computer account for the new Domain Controller has been created in the domain.
2. Verify the Domain Controller is running the latest updated versions of the software.
3. Check that the correct Domain and Forest Functional Levels are selected.
4. Check for any mis-configurations in the Active Directory replication and make sure it is in sync across all Domain Controllers.
5. Verify the domain controller is able to communicate on all the correct ports.