Trying to reset my security questions but received the message that there is not sufficient information to reset…what should I do?

So it appears that someone is using my old Apple ID as I’m getting messages daily of new devices logging on. This old account is secured with security questions and my answers are not matching although I’m pretty certain they’re correct. When I attempt to change the security questions, I receive a message from Apple stating there is not sufficient information to reset the security questions. What should I do now?