Hello,
Thank you for posting in Q&A forum.
This issue may caused by the AAD connectivity.
You can login to Azure Portal by admin account and go to Azure Active Directory,. In Connectivity Status or health selection, you will be able to check the AAD connection.
Meanwhile, you can also try to open AAD connector, check the configuration and sync on-premises AD and AAD.
To help other customers who may be facing the same issue, please don't forget to vote if the reply is helpful.
Hope this answer can help you well.
Best regards,
Jill Zhou