I tried "If I want, I could!", and it rejected "could".
Sometime I've seen "could" accepted in this course in similar sentences, and it seems to me that "could" and "can" could / can be used with equivalent meaning in English.
Are these meanings not close enough together in Esperanto?
OK, a few lessons later, I came upon a similar thing, and when I tried "could" again, it rejected it, but gave me one of those informational popups saying "povas = can, povus = could". So I guess that's the difference.
Maybe that popup should be added for this translation as well.