Hello PhantomCordShinobi,
Thank you for posting your query in the Microsoft Q&A Community.
There are a few bits to check when encountering authentication issues while trying to domain join machines to Entra ID managed domain.
- Make sure this is not a guest user (external account) invited to the tenant. Only users created within the Entra ID tenant or part of the managed domain should be used to complete this process
- Check that you have enabled password synchronization to your managed domain. Without this configuration step, the required password hashes won't be present in the managed domain to correctly authenticate your sign-in attempt.
Follow the link to get more information about how to troubleshoot authentication-related issues in a managed domain.
Let me know if further assistance is needed.
Babafemi