Hello,
I hope you are keeping well. I have seen this error in the past and I believe it is related to these two settings which should be set to true.
Authentication/Registration/ExternalLoginEnabled
Authentication/Registration/Enabled
If you are still having issues, I recommend opening a support case for a proper investigation.
I hope this will resolves your issue. Have a good day.
Thanks,
Mark Biggerstaff
Dynamics Support