"A lion is not a dog."
Translation:Lew nie jest psem.
While of course it is possible that there really was a bug, usually the answer is that the learner either mixed the forms (for example wrote "Lew to nie psem" or "Lew nie jest pies") in an ungrammatical way. There's also a possibility that there was a typo you haven't noticed. So without knowing what you actually answered, there's not much I can say.
The two options for saying "[noun] is (not a) [noun]" are:
[Nominative] + to (+ nie) + [Nominative] = "Lew to (nie) pies".
[Nominative] (+ nie) + jest + [Instrumental] = "Lew (nie) jest psem".
You used the Instrumental form "psem" after the word "to", which is not an option, your sentence is ungrammatical, I'm afraid.
More info here: https://www.duolingo.com/comment/16373167