Well, the genitive would be девушки, but I have the same question as you. Can someone answer greenstreet365's question? Doesn't the negative call for genitive??

Wouldn't using the genitive here be like saying Jenny isn't any girls, instead of sayi g she isn't a girl? Normally when I see the genitive used with a negative it's saying there isn't any of something.

because it is nominative :) (we ask "who?/what?" about nominative)

That would be as nonsensical as "Jenny no girl".

Девочка refers usually to a young girl, like a child. Девушка is more like a young woman or can also refer to a girlfriend.

Но is used when there is a meaning of "despite". Учитель зашёл в класс, но дети разговаривают - The teacher entered the classroom, but kids are talking. Он не учил песню, но знает слова - He didn't learn the song, but he knows the lyrics.

Well, "Jenny" is a female name, so one would assume we are talking about a female cat - "кошка". Not that it's impossible to name your male cat ("кот") Jenny, but I don't blame the creators of the course for not anticipating this answer.

Just found this and now I understand: https://www.duolingo.com/comment/11536858/A-Guide-to-Using-%D0%AD%D0%A2%D0%9E

I want to know why you'd use это here at all.

On returning to this exercise, since I got it wrong at first, Duo accepted Дженни не девушка, а моя кошка.