I agree that this variant should be accepted. "We have no ..." and "We do not have a ..." are equivalent in English, and it's standard in Duolingo courses to type the latter version because it's shorter. I can't see any reason specific to Latin to handle this differently in the present course. Nevertheless, "We have no courier" isn't accepted, either. So either we are missing a detail or there is some serious pointless pedantry going on here.