We don't say "a enormous cake," but rather "an enormous cake," as "enormous" begins with a vowel. Marked wrong. К сожалению, Duolingo has several of these errors here and there.
Report it and they'll fix it. The course is still in beta, of course there are a lot of errors still.
Why not "Let's buy the huge cake!" as if there were cakes of many sizes, but we just wanted to buy the big one.