"The shark eats the fish."
Translation:Lo squalo mangia il pesce.
If you mean that you saw the sentence in english and translated it into "Lo squalo mangia i pesci", then I'd say that's correctly translated. Without any context, you can't be sure if it was talking about just one fish or more than one. So it should be accepted. Next time you come across it report it.
It seems that the program randomly recognizes certain typos and will mark your response correct but point out that you made a typo and for others it'll mark the response wrong because of a simple typo. I just got this wrong because i wrote "lo dqualo" by mistake instead of squalo.