[SR-Users] Forbidden - Private identity not found (Authorization: username)

Hello dears when i was trying to build an IMS platform by using kamailio with sip digest authentication scheme and ims_auth module, i couldn't register and the result was as following : 1-the first register message passed to the s-cscf then S-cscf replaying with the message “401 unauthorized - challenging the ue” 2- the second register pass to the S-cscf with the authorization header then the S-CSCF replying with "Forbidden - Private identity not found (Authorization: username)" So based on that could any one tell me what is the reason of this error and how to fix it ?? Thanks in advance