In spoken English, a request for permission "May I ... " is generally answered with can, cannot, or can't OR with may or may not - even if the question was formed using may.
"Yes, you can." or "No, you cannot / can't!"
Which isn't a modal for ability? must / be able to / can\can't / could\couldn't
must
Which isn't a modal for permission? may / be allowed to / had better / can
had better