Could be your engine and/or clutch.
I'm guessing it's a China Girl/Happy Time setup
.
Raise the rear tire off the ground.
Turn the pedals.
Still jammed?
Loose your rear wheel brake cable. If that loosens the rear wheel, that's your problem to fix.
Still jammed?
Remove the bicycle chain.
Still jammed?
Squeeze the clutch cable as tight as you can, and spin the left pedal with your right hand. If that loosens your rear wheel, your clutch cable/adjustment is the problem to fix.
Still jammed?
With a wrench or your strong hand, push that tough clutch lever inward and spin the left pedal with the other hand. If that loosens your rear wheel, your clutch/cable adjustment is the problem to fix.
Still jammed?
Find the master link on the left-side engine chain and disconnect it.
Spin the foot pedal.
Do the pedals spin freely?
No?
Then the trouble lies in the bearings between the foot pedals.
Do the pedals spin freely?
If yes, spin the rear tire. If it won't spin, check to see if there's a 2 by 4 lodged between wheel and frame.
J/K. If nothing's visible, maybe your rear wheel bearings are shot?
If the rear tire spins freely, your engine or engine sprocket is to blame. Check to see if there's a 2 by 4 lodged between engine sprocket and engine case.
J/K about the 2 by 4, but there MIGHT be something jamming the engine sprocket.
If the rear wheel spins freely, reinstall the bicycle chain, but leave the engine chain off. Pedal the bike to the man who sold you the bike.
Rejoice in the fact that you tried to fix your bike, and you learned more about MB's and bicycles in general.
Good luck.