Yes, only a limited number of advance tickets are sold, and this usually means that, if your intended visit is within the next 4 weeks, all availability is gone.

You don't need to have advance tickets, though. You can line up (there is invariably a queue for walk-up tickets) but bear in mind you could be in line for up to an hour before you get inside.

If you don't fancy waiting, there are lots of other things to do. Just stroll around the city, visit any of the other museums (the Royal Palace on Dam Square is not far from either Amsterdam Centraal, or from the Anne Frank Huis) and would easily take up an hour or so of your time.