I have a question

Which one is correct?
"why can't I sleep" or "why I can't sleep"

Both are correct without any context.  However, if you are asking a question.  The first one is correct.

The second one is correct if you have more written after it....

Why I can't sleep is that I had drank a cup of coffee not too long ago.

I am assuming you are talking about the correctness of grammar structures.