na jeji obliceji?
na jejím obličeji ^
"Obličej" is Masc InAn, "tvář" is Fem.
why can't "obličej" be used here?
You can. All accepted forms contain it as one of the variants.
I wrote "co je na jejim obliceji", why is it wrong?
You are missing some translation of "that". Your translation just means "What is on her face?".