If you are withholding food, I'd say she isn't eating enough and you need to feed her daily. She might not have poop because she doesn't have any, literally.
If you are offering food but she isn't eating much, maybe there is a blockage?
Do you soak the tortoise in warm water? Does the tortoise have damp substrate and a water dish? Could be dehydrated if these answers are a "no".
I'm not sure about the eating habits of star torts, but not going in 20 days definitely sounds concerning. It is still active or is it lethargic? I've read that some can go around 2 weeks before "going" but I think this may warrant a vet visit.
How much does she eat? How warm is she? If she is eating and warm enough to digest her food, she should be pooping. Any chance she is eating the poop before you find it?