You can say just sum femina or femina sum, if it doesn't accept them report it.
Having the ego give more emphasis (like how you may say: I am the mother[, not you!]).
Ego sum femina still sounds redundant. It's a pretty rough way of translation without considering the standard Latin word order and omission (or earlier, non-existence) of personal pronouns. When you're asked to translate "I am a woman", all the versions should be accepted, but when you're asked to translate it as "I am a woman" from Latin, the sentence in Latin should be "Femina sum" because that's the basic way of saying it and thereby the users can get used to word order. It is like that in all other languages, particularly Romance, all over Duolingo.
Redundant does not mean false. I see that you have learned a bit polish then you surely know that both "jestem człowiekiem" and "ja jestem człowiekiem" are both correct although they may have a bit different meaning.
If Duolingo does not accept sum alone, then that's an error. If it is more of a general question regarding the language... languages are not made, they evolve, so many things are just not logical. However, regarding ego, Moopish sums it up nicely.