When a namespace is first set up, credential information is created first for the administrators who will support the domain. Once they have credential information, they can create credential information for other administrators, client machines, and client users.
When you try to create your own credential information, you run into a problem of circularity: you cannot create your own credential information unless you have Create rights to your domain's cred table, but if the NIS+ environment is properly set up, you cannot have such rights until you have credentials. You have to step out of the loop somehow.
You can do this in one of two ways:
By creating your credential information while logged in as superuser to your domain's master server.
In either case, your credential information is thus created by another NIS+ principal. To create your own credential information, follow the instructions in Creating Credential Information for NIS+ Principals.