"Minha comida principal"
Translation:My main food
Well, "my main food" just doesn't make much sense. If comida principal would be the portuguese way of saying "main meal" (though not literally so), I too believe it should be accepted.
We are used to call "comida" the actual food that is on the plate.
And "refeição" the meal or the act of having a meal.
So, if we say "minha refeição principal", we are talking about the main meal of the day.
If we say "minha comida principal", we are talking about a particular type of food. Like "rice is my main food" (arroz é minha comida principal)