Thank you for reaching us!
Could you please explain and share the steps you have followed, and the error screen shot for better understanding and assist on your issue.
Could you please try to login the new user account by using https://login.microsoftonline.com/{tenant ID} and let me know what you are getting.
if the Identities property value show as a mail the user has signed up by using Microsoft Entra External ID email one-time passcode (OTP).
Also, Review the Conditional Access Policies to see if any are restricting access for the user. verify Restrict User Access to Entra ID admin Portal which as shown in the below image.
Hope this helps. Do let us know if you any further queries.
Thanks,
Akhilesh.
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