You have probably forgot to add privileges for the user, that you are trying to register with, to perform authentication with the algorithm of your choice. In most cases you switched the authentication method in the S-CSCF but you did not allow the user to use anything else than AKAv1-MD5.

This error might also indicate that you are trying to use an authentication scheme not supported by the HSS in use. Please check that the authentication scheme is supported.