I think that would be πάνω από το πιάτο rather than πάνω στο πιάτο. (Literally, "above from the plate" rather than "above at the plate", perhaps?)
I put "my apples are on top of the plate" which was marked wrong. Is it correct?
Well, I don't think that sentence makes much sense in English. On top of could replace on in some cases, but they are not interchangeable, and it would sound odd in a sentence like this one.