I entered "this human is eating an apple" just to see if it would accept it. It did not. I wonder if even "this person ..." would have worked. I think it is technically completely correct though, like olimo said.
Unfortunately the Russian course is hugely lacking in that the tasks reject many alternative answers that are correct, even the "does/is doing" thing is always a problem, it sort of steers you to write unnatural English because once you know the pitfalls you tend to just avoid the ones you know.
For example the German course understands some fairly complex transformations such as "everything is balanced" -> "alles ist in dem Gleichgewicht" ("balanced" is either an inflected verb, or an adjective, "das Gleichgewicht" is a noun). Of course there are substantial differences between the languages but sometimes, because it seems to be taking years just to add additional correct translations, I wonder if the Russian team is too small for the amount of work needed or too busy to correct stuff? I wish there was a way to help :/
Not if you mean the fruit. 'This man eats an apple' is correct, or the plural 'eats apples' could be used without an. 'This man eats apple' could be OK if it were an answer to a question about apple flavor like, "Who wants cream pies? I have peach, apple, and strawberry."