Hi @Smart Cut
Thank you for posting this in Microsoft Q&A.
I understand that you have converted an external admin user to an internal user by clicking "Convert to internal user." After the conversion, you are unable to log into the Microsoft Entra Admin Center.
External user conversion is a feature in Azure Active Directory that allows you to convert an external user account to an internal user account without the need to delete the existing user object and create a new one. This feature is useful when you have external users who need internal access to your organization's resources. By converting an external user account to an internal one, you can preserve the user's original account and avoid disrupting their access.
Could you please confirm the type of external user? Is it a cloud user or a synced user?
When a cloud user is converted from external to internal, administrators must specify a UPN and password for the user.
Use Microsoft Entra Connect to synchronize your on-premises identities. When you convert a user from an external user to an internal user, Microsoft Entra Connect synchronizes the user's attributes to Microsoft Entra ID, so the user is managed as an internal user going forward.
Please try logging into the Microsoft Entra Admin Center using the new credentials and let me know if you're still encountering the same issue.
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
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