'Can I not go?' is ambiguous. The person might want to go and is asking in the hope that they will be allowed to go. However, it could also mean that they don't want to go and are asking to be allowed not to go. It would dep[end on the context and the word stress. If the emphasis were on the word 'not', then I would say that the second meaning is more likely. If I saw the sentence without further context, I would take the second meaning as the more likely one.