"Who is she?"
Whenever this question is used, it has to be clear by some means who is being referred to, and "she" doesn't really pin that down either, in general. So it seems perfectly reasonable for Duolingo not to require an indication of gender that's not, in general, necessary in Japanese.
Similarly, I'm sure the Spanish course would accept "¿Quién es?" for "Who is she?"
We can't use が for 彼女 because the information we want to know is 誰, 彼女 must be a topic in this sentence.
As a rule of thumb, when choosing は or が for the subject, if the question word is after the particle, the particle has to be は. If the question word is before the particle, it must be が.
Your sentence essentially translates to "Who is her?", which is grammatically incorrect. In English, we commonly switch the word order of subjects and objects when using a question word like "who", but even though we say "Who" first, "She" is the subject still (it would turn into "her" if it was the object). Since かのじょ is then the subject, it needs to get the は, and the object, だれ, needs to preceed ですか.
There are three answers, and two of them mean the same thing: 彼女、だれ？ and かのじょはだれですか？
I chose the first one and it marked me wrong. The only difference between the two is the second is more polite. The question was to translate 'who is she', not to choose the most polite way of saying it.