Someone has been very pedantic with the translations for this skill. I think "coffee, please" or "coffee, thank you" should be accepted as correct in this situation. Also in the same skill writing "yeah" instead of "yes" on a certain question is considered wrong. A bit unnecessary I would argue.
In english one can order (a) coffee by simply saying "coffee, please". How would I say that in Finnish? If were a betting man I'd say that you'd say "kahvi, kiitos", right? Therefore "coffee, please" should, in my humble opinion, be accepted. Please correct me if I'm wrong.
The 'problem' here is probably that 'coffee, please' would translate more to 'kahvia, kiitos'. So while 'coffee, please' might be more natural to say (some people in the forums actually argue it's more natural in Finnish to say 'kahvia, kiitos' as well), the fact 'kahvi' is used in stead of 'kahvia' tells us it should be 'a coffee' instead of just 'coffee'.