"He eats a steak."
Translation:Ele come um bife.
Perhaps the reason it wasn't accepted is that omitting the pronoun in this case has introduced too much ambiguity. If someone tries to back translate your sentence, which English pronoun would they choose: "it/he/she/you" or even "we" (because "a gente" is also a 3rd person singular pronoun)? Only by explicitly writing "ele" can you be sure they will choose correctly.
Without the pronoun your sentence could also be interpreted as a command because "come" is the imperative for "tu".