There is a single "admin" user with "root" access. The name by default is netscreen but can be changed.
This is the only user that can run this command. Other admin users can have full read/write but cannot change the root user or create more users.
In the configuration look for this line giving the name of this root user netscreen is the default as listed here.
set admin name netscreen
This is the user you need to log in as to create more users.