I'd never charge my child rent to live at home.
However, you've now got a situation, as you've said she must and she won't. 🤦♀️
You need some compromise now so you don't feel fucked off with her and she doesn't get away with refusing.
You need to take control of it. There now needs to be a deal. Eg, ok, I'm sorry, it's not I wanted to make money out of my own kid or be unsupportive, I just wanted to make you understand about being responsible for your own living costs. But let's say instead we agree a few things you'll do to help out. (Then think of some)
The real issue is that she struggled in college and therefore is now having to work. She didn't choose to get a job - she was failing at being at college, which no doubt made her feel awful. Now she knows if she'd not struggled at college, she'd still be there and wouldn't have to pay rent at home. So basically you charging her is like you punishing her for not being 'good enough' for college. And presumably she has friends still in education and not paying.
Personally I'd be bolstering up anything successful she does, and I'd say don't worry love, you save your hard-earned money.