It all really depends on the theory of translation you hold to. Duolingo tends to lean towards the more literal side of things. That being said, strictly speaking, it wouldn't be correct to translate the sentence this way.

I believe this is a possible (and, in fact, more common) translation of the phrase, but I'm not sure this course would accept it.

I wrote "they never see anything"

It is common in many languages that double negatives do not cancel each other.