ORA-28020
The certificate thumbprint was specified incorrectly for the external user.
Cause
Either the syntax of the IDENTIFIED EXTERNALLY AS clause is invalid or the format of the certificate thumbprint is incorrect.
Action
If you are creating an external user with a certificate thumbprint, the IDENTIFIED EXTERNALLY AS clause should have the format: "IDENTIFIED EXTERNALLY AS dn WITH THUMBPRINT <hashtype:xx:xx:...>". Please check the syntax and the thumbprint format.