"You can't smoke at the airport."
Translation:No puedes fumar en el aeropuerto.
I won't try to answer how well your alternative works or if there are issues with it. I'm not sure. But I will note that I think you did not succeed in using the formal, which I think is "usted puede", but the second person plural, ustedes pueden. I will leave it to native speakers to address if that works.
I used what I think is the more common way to put this, "No se puede fumar en el aeropuerto," and it is now also accepted by Duolingo. This avoids the issue of familiar vs formal language and the issue of plural and singular. I learned to translate "No se puede..." as "It is not permitted."