GRAMMAR QUESTION

"Is something cannot let me know" does not make any sense. It is hard to understand what you are trying to say.

I mean that someone says something, but they don't wanna let me know. How do I change it to be a question.

Please don't use "wanna." It immediately lowers the reader's opinion of your writing.

We still don't understand what you are asking. Is there a statement that you want to change into a question?

Are you looking for something like this? "Is there something you can't tell me?" or "Is there something you don't want me to know?"

yes , I mean "Is there something you don't want to let me know." could I say "Is something you cannot let me know?"

No; you need 'there'.