Translation:I have never seen this textbook before!
Why couldn't it also be "I have never before seen this textbook"? The word order doesn't sound wrong to my American ear.
Nearly: noch nicht = not yet (or as in your sentence: not before).
ich habe es noch nicht gesehen = I haven't seen it yet.
Duo doesn't accept "I have never seen this textbook yet". Unfortunately, I read comments, after answering without "yet", and cannot report it.
Why not? This looks like the most accurate word for word translation that carries the same meaning.
It does not carry the same meaning; noch nie is “not ... ever” or “never” in combination with the present or past perfect, and translating it with “still” is not appropriate in my opinion. (In fact, “I have still never seen the textbook” simply sounds wrong to me in English.)
Person 1: "Last time I talked to you, you said you had never seen that textbook. Have you gotten a chance to see it yet?" Person 2: "No! I have still never seen that textbook!"
How would you convey "I have still never seen that textbook" in the context of the second person's speech (in the example above)?
the combination of "still" and never" sounds a bit strange to me, although I understand your example. Nevertheless I would phrase it "No! I have still not seen that textbook" and in German "Ich habe das Lehrbuch immer noch nicht gesehen".
Why does Duolingo not accept "I haven't seen this textbook yet."?
That would be noch nicht gesehen rather than noch nie gesehen -- the emphasis of "never" is missing when you merely use "not".
Opposite of noch nie is schon mal, right?
Hast du dieses Lehrbuch schon (ein)mal gesehen?