There's some states that make it illegal to sell your car or other vehicle without the title but some states you can still sell the car without having a title to it.
But the person who buys your vehicle will not be able to register the car or get it in their name without the title.
If you used to have the title but lost the title to the vehicle you can go to the DMV and put in for a lost or duplicate title so you can sell the car with a title.
If you still sell the vehicle without a title some other paperwork will be required when selling the vehicle and one of those documents would be a Bill of Sale.