Go to the airport anyway. Most likely you will end up with a seat. If not, the airline will take care of you and get you to your destination on another flight.
Some possible explanations:
The flight is overbooked. If someone else fails to show up for the flight, you'll have a seat. If not, they will offer incentives (vouchers for future travel) for someone to give up their seat and take a later flight. If nobody does, they will bump someone at random (not necessarily you), give them compensation, and put them on a later flight.
There are seats still available, but they want to save them for special customers (elite-level frequent flyers, etc). Perhaps they are "premium" seats with extra legroom, exit rows, etc. Right now these special customers have seats elsewhere on the plane. Sometime before departure, the airline will upgrade their elite customers into these premium seats, leaving their original seats for you.
So you most likely will get a seat, though you may not have much choice as to which seat.
If you buy your ticket further in advance, when more seats are available, you will have a better chance of being able to confirm your seat assignment.