The [privilege page](http://superuser.com/privileges/protect-questions) states that the question must be at least a day old. However the screen shot indicates that the question is more than a day old so it should be there.

I've noticed that your rep is *exactly* 15,000. Perhaps there's a bug in that the privilege itself checks for `rep > 15000` while the notification checks for `rep >= 15000`.

Most of the time you'd be **over** the limit rather than **on** the limit so you wouldn't notice the error.