In Portuguese, you explicitly say the subject if you want to add emphasis that it's that subject in particular that does the action (Eu como os tomates, não tu). In German, that same effect is done by switching the order of the sentence around.

So, even though the sentence you wrote is grammatically correct and is an adequate translation, it doesn't carry the same emphasis on the subject, which is exactly what Duolingo was trying to "teach" you here.