"A tall girl and a short girl."
Translation:Una niña alta y una niña baja.
Because most of the time the adjective comes after the noun. There are some exceptions. https://www.spanishdict.com/guide/adjective-placement
"chica" is accepted. If your answer was marked as incorrect, the reason was elsewhere. It is always best to share your full answer in the forum so it can be completely checked.
I'd love to see a screenshot of what you submitted along with Duo's response so that we can try to see what went wrong.