Me either? Is there such a thing?
Yes there are lots of red grasses. But I bet it was intended to be one of Duolingo's weird, therefore stick-in-your-head, sentences!
Why not 'tidak pernah'?
tidak pernah means something that a person has never done and will never do in the future whereas belum pernah implies that they might do that thing one day.
This sentence could be translated as "I have not yet ever seen red grass"
I haven't ever seen red grass?
Yes that is correct English.