"He can cook."
Translation:Он умеет готовить.
уметь specifies the ability to do something. Он умеет читать = He can read (he has the ability to read, he knows how to do that); Он может читать = He can read (because he has the time, he's not busy).
Without more context, он может готовить should probably be accepted. It's not too hard to come up with an appropriate situation: "he has an hour today to do something, what can he do? He can read a book, he can wash his clothes, he can cook." Usually you'd specify what he is cooking, though, and not just the action itself.
If you wrote it in the English alphabet, then it transliterates to Он умеет готовит, which is improper verb conjugation. Готовит means he, she, or it "cooks." In this exercise, the verb is in the infinitive perspective "to cook," so it must be готовить. That "Ь" must be at the end of the verb to be correct.
It's always better to learn Russian in the Russian alphabet in order to avoid this type of problem. I don't know if Duolingo understands an apostrophe (') in the English alphabet as a transliteration of the Russian "Ь"
готовит = gotovit
готовить = gotovit' ??