[Plugin: O3World Members-Only Categories] Not Found Message not good

this is entirely by design.

to designate a category as a ‘private’ category,
you do so on the Privacy settings page.
as such, you must be an Administrator to do so.

to assign a user to a category that’s been set to ‘private’
(and thus, ‘grant them access’) you must be able to Edit users.
typically only an Administrator can do that.