What is it about Duolingo and genders? Half the masculine nouns beginning with a vowel seem to miss the initial "t-". This should be "an t-arán" because it's a first declension masculine noun, shouldn't it? This seems to be a recurring error.
Could it be that the "t-" only gets added when the noun is the subject or when it stands alone? In this case "arán" is the object. Though other folks here have already written that Irish no longer bothers with an accusative case. It's puzzling.
"an arán" is simply wrong here. "an t-arán" is always correct (unless preceded by preposition).