"Где аэропорт?" can be used in both cases: 1) When you ask about any airport (you are a stranger in the city and just want to fly out) 2) When you ask about the certain airport (you have to meet a person there)
"Где этот аэропорт?" can be used only in the second case.
I guess you already noticed, but for future readers:
When writing down the "write what you hear" sentences, small mistakes are still accepted as in the other lections, but are not shown due to the "Well done" message