why is "je cukr v jablkach" incorrect?
This word order is more for a question, not a declaration. I can't imagine it not even as a response, where you often can repeat the words of the question with a different intonation.
Why is "Cukr je v jablkách" incorrect?
I would also like an answer to this question.
That is the answer to "Where is (the) sugar?". Or if not an answer it just informs where (the) sugar is. It is not informing that there is sugar in apples.
It is the topic/comment distinction https://en.wikipedia.org/wiki/Topic_and_comment