"A human is an animal."
Translation:Człowiek to zwierzę.
I don't know what to say, it's a perfect answer, it should have worked. We'd need a screenshot if we are to investigate.
Well, this is how Duolingo works, at least right now. Did you see the Tips before starting this skill? In the browser version when you click on the skill icon there's a button "TIPS" and some reading material with explanations.