She still can. You probably don't fulfill requirements. With exhibitionist origin she can do it from the start. Or you've encountered a bug
It's not in a wardrobe. You can toggle panties and stockings (separately) in a drop-down menu after pressing arrow up above pound symbol - the one you press to work as a barmaid. Perhaps you don't see it because your character won't agree to work pantyless. If you don't see it try creating new exhibitionist run real quick, I guarantee you there will be an arrow above pound symbol at a bar.