Hi @Anonymous , if you prefer this method. To avoid the error what you are getting
By Admin:
1. Login to https://portal.azure.com/
2. Click on Azure Active directory > users > Add guest users
3. Add their email id as invite user

4. Once the external user email address has been added as guest
Then do the portal sync configuration. Then external users can login with Azure AD.
Note: As @UlrikkeAkerbæk Microsoft preferred way is Azure B2C. But the solution I have mentioned also works.
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