In English there are two indefinite articles to use: "a" and "an". Which one you should use depends on the sound that comes after it. If it's a consonant sound or a "long u" sound, then the article should be "a".
a door ('d' sound)
a yellow car ('y' sound)
a photograph ('f' or 'pf' sound)
a unicorn (long 'u' sound)
Else, if it is any vowel other than a "long u" then the article should be "an".
an open door (long 'o' sound)
an elevator (short 'e' sound)
an ultra crazy surfing trick (short 'u' sound)
an airport (long 'a' sound)
Hope this helps you. :)
Edit: I had just realized that Duolingo gave "el aeropuerto" for us to translate. You should have used the definite article "the" instead.