assign user to course problem

Please help. My moodle 3.0 always return error when I want assign course to user.
I did steps by this wizard: http://www.rumours.co.nz/manuals/using_moodle_web_services.htm

I can list courses, list user, add user, but I can not assign course.

Always I got error:

Array
(
    [exception] => moodle_exception
    [errorcode] => wsusercannotassign
    [message] => You don't have the permission to assign this role (5) to this user (6) in this course(2).
)