Duolingo needs to decide if putting "a" in the translation is right or wrong. Here,I got it wrong for leaving "a" out. On another, I'm wrong for putting "a" in.
it depends on the sentence in Arabic in the first place. The phrase (رجلٌ فرنسي) has no AL (i.e. not الرجل) and hence the word is indefinite. Since it is indefinite, then it must have a/an in English.