It is an exception. There are some very few masculine nouns that, as an exception to the general "е" ending, use a "у" ending with "на" and "в". Some are taught in the lesson. See the tips and notes of the lesson "Around you".
Notice that most of these exceptions are monosyllabic, like "порт", and аэропорт "inherited" the already existing "порт" exception.
Yes, I should pay more attention to the Tips & Notes. After I got this wrong, I looked back and found that Мы в аэропорту is an example in this module's T&N.
Wade's A Comprehensive Russian Grammar has a list (not complete) of common words declined according to this pattern. At least 40% of the words on the list are in this Duolingo course.
О is curiously pronounced as а in stressed positions in rapid speech in many of the words here.
This course has so problems with in/at, if I may say. I never see "at" in the answers, but it's enormously used in English (i.e. being "in the park" in Duolingo should really be "at the park").