Translation:The animal is neither a cat nor a dog.
the animal is neither cat nor dog is an exact translation of the greek sentence and should be accepted.
You did not use an article before cat or dog in the greek sentence. In truth in english one could say this either way but I chose to omit the a because you did not use it in the greek and it was marked wrong.
"the animal is neither cat nor dog" is marked as incorrect but I don't think we need the indefinites in the English translation
English is very strict in its word order and also depends on a lot of fixed expressions. You'll find those in the translations. That's the best way to learn, by using what native speakers us and not trying to translate from Greek.