Check engine light code P0720...failed output

The output shaft speed sensor lets the computer know how fast you are going down the road to help it pick gear change points..
Yes, you can buy one and replace it.
First check the wires and plug going to it.
Your manual will probably describe its location better than me, but it will be toward the rear of the gearbox and usually has a small gear on the end that meshes with a mating gear on the output shaft as you put it in.