Translation:When you finish the meal, you can eat cake.
It's another one of those unnatural sentences, I think. Again, this might be caused by the differences in American and Australian English. I would be interested to know if any Americans think that "finish your meal" is more often used than "finish the meal". To me, it sounds strange to say "finish the meal" since the only meal you are likely to finish is your own.
I understand about the different translations, but every time I see the phrase "finish the meal" in that particular sentence I wonder which meal. It needs context for me. For example, your mother went to a lot of trouble to make dinner. You can't just have cake. When you finish the meal, you can eat cake. Now the sentence is fine. Without context, it sounds really unnatural. Obviously, you can't put context in the exercises, and I'll just have to get used to it.
"Pasta" is the object of the sentence, but it is does not have the accusative marker "ı". For that reason, you cannot write "the cake" because in the Turkish sentence it is an indefinite direct object. You would only use "the" when the Turkish word has an accusative marker on it. In English, "cake" and "some cake" have approximately the same meaning, so you can use it in the English sentence even though it does not appear in the Turkish sentence.