Not clear what you're wanting to do? If graceful demotion does not work then you can turn off the failed one, seize role (if necessary) to another healthy one.
then do clean up to remove the remnants from active directory.
Clean up Active Directory Domain Controller server metadata
Step-By-Step: Manually Removing A Domain Controller Server
--please don't forget to upvote
and Accept as answer
if the reply is helpful--