User can't assign category to listing

When a user adds a listing in the admin panel, they cannot assign a category to that listing although the list of categories are shown on the right side of the screen. I've tried to change the Users capabilities a few times but this option, to assign a category, is not among the options. I can only assign a listing to a category in my main site as the admin, not user. Pls. help. I feel like I'm going in circles with this thing.