I'm glad that your issue is resolved and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others Opens in new window or tab", I'll repost your solution in case you'd like to "Accept Opens in new window or tab" the answer.
Error when trying to make custom domain primary in Microsoft Entra ID
I added a custom domain to my Microsoft Entra ID and verified it with a DNS TXT record. However, when I try to make the custom domain primary, I get the error "Unable to make domain name 'mydomain.com' primary of Default Directory." I have a non-federated domain and no additional error details are provided. Any suggestions on how to resolve this?
when you check a day later as result it had converted the domain to primary domain.
If you have any other questions or are still running into more issues, please let me know.
Thank you again for your time and patience throughout this issue.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.