"Do you not see a cat?"
I wrote 'nevidiš kocour' and was marked incorrect. Not sure how many 'cat' alternatives to accept, but I was mildly annoyed.
"Kocour" is fine, it's a male cat, but you need it in the accusative for this sentence to be correct. -> "Nevidíš kocoura."
It is "kočku" (in the accusative, as you said), because it is the direct object in this sentence.
Here in Duolingo is both of them correct: Nevidíš kočku and Nevidíš kočka. I have thought, only kočku is correct = Akkusativ. ??