Managing external identities to enable secure access for partners, customers, and other non-employees
Thank you for posting this in Microsoft Q&A.
I understand that you have encountered a login issue with Microsoft Entra ID for customers.
AADSTS500208 is a general error caused when the user's account doesn't match the expected account type for the given tenant.
1.Ensure that you are using the correct account to log in.
Customer accounts supports to sign in with a local or social account.
2.Confirm the application you are trying to log into.
If a customer account is trying to log in with their Microsoft Entra ID, they might receive this error because customer accounts do not have access to tenant resources.
Please refer below documents:
Default user permissions in customer tenants
Supported features in Microsoft Entra ID for customers
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya