"You are men and I am a girl."
अनुवाद:आप आदमी हैं और मैं एक लड़की हूँ।
Duolingo likes you to translate sentences keeping their exact original meaning intact (this is probably a good idea in any translation!).
The English sentence we are translating here is "you are men...". If try to use "आप सब..." here we are saying "you all..." - which gives it a different meaning! Imagine a room with 2 men and 100 woman, we point at the two men and say "you are men" but we can't say "आप सब..." here. So "आप सब..." can't be correct on Duo because it changes the meaning!
You could use "आप लोग..." instead of just आप and that (in my opinion) would keep the meaning the same (I'm not sure if it is accepted as a correct answer though, I will try to check for you).
Thanks for posing the question and thanks to _gs9 for answering it so nicely.
Together, you have illustrated the power of the sentence discussion part of this forum (चौपाल). This is the exact purpose of having a sentence discussion so that future language learners can come here and have their doubts cleared.
Let us try to keep the non sentence related questions out of here.