Of course there is a thing as portakallar :)
But often it cannot be used as an object of a sentence. It is totally fine to use it as a subject or predicate.
- Portakallar masada. (The oranges are on the table)
- Masada portakallar var (There are oranges on the table)
- Bunlar sağlıklı portakallar (These are healthy oranges)
"The girl" in this sentence is the agent and "the orange" is the accusative object.
"Girl eats the orange" would be grammatically incorrect in English without the article so you need to include the article. Not because "the girl" needs to be in accusative case in the Turkish sentence but to follow the rules of English grammar. I hope this helps.
Why is it not kız portkalı yiyor? Doesn't "yer" - form mean that she in general eats oranges, as opposed to being allergic to them for example? If portakalı is in accusative form then we are speaking about a specific orange, and therefore it is an action being comitted now that we speak about, then should one not use "yiyor"?