The interviewer wants to know that you can knowledge weaknesses, because when you know something is a weakness you are able to work towards improving it. You don't have to tell the interviewer about personal weaknesses. If you can't think of a weakness, think about conversations you've had with an old boss during review time.

If you are having a hard time coming up with a weakness, you can think about something you'd like to improve on. When answering this question remember to talk about how you're working to fix the weakness.