"Le garçon a une veste chaude."
Translation:The boy has a warm jacket.
While this is technically correct (and you should submit it to see if it will be accepted), it seems unlikely to me. If the speaker wanted to stress that the boy had only one warm jacket, they would say that. Ex: "le garçon a seulement une veste chaude." However, when the French use "un/une [nom]," the un/une is almost always intended as an indefinite article, and not as a numerical adjective as it might be in English - i.e. "un/une" translates more directly to "a/an" than it does to "one" in this situation. That's what I would guess Duo is trying to teach. There would have to be context to prove that it was being used as the latter, and there isn't.