This is how I understand this. In Poland there is no such thing as a 'cheque' as we have in the UK. I know this as I have helped a number of Poles who have been paid with cheques for various things but don't know what to do with them. In e-mails they always called them a czek. Rachunek as you say is a bill, usually in a resaurant, and paragon a reciept. So I doubt if cheque is a correct translation of rachunek.
Yeah, it isn't... but it's accepted automatically. Because "check" is a correct translation, especially in the restaurant context, and then Duolingo accepts "cheque" as a British variant of this word, not taking the meaning into consideration :/
True, I don't think I've ever seen anyone write a cheque in Poland...