What is the seal called that is on the drive shaft that turns the front sprocket

did you turn the locks on the wheels, or check the fuses

Probably a seal that is around the shaft that the sprocket is attached to. You need to remove the chain and then the front sprocket to check the condition of the seal around the drive shaft. See if it looks like oil is coming out around the edge of the shaft. It is best to get a service manual for your specific bike and follow the instructions. It's not that hard. You start at 1, then go to #2, then #3, ...

This could be a few things:
1) Your clutch plates may have worn out so that even when the bike is in gear, it will just rev, but no power will be transmitted to the drive shaft (smaller front chain sprocket is the driving gear, larger rear sprocket on the back wheel is the driven gear)
2) The front chain drive sprocket has come off under the cover on the motor - if the chain is really slack, have a look.
3) The front chain drive sprocket SHAFT has sheared. (basically, meaning that the shaft has broken into two parts as it couldn't handle the drive power (torque) being delivered through it to the chain drive) With the motor on, listen for grinding noises internally
when you rev, from the vicinity of the front chain drive cover. If its rough and grindy, the shaft may be sheared. Then with the motor off, take the cover off and leverage the small chain drive sprocket away from the motor with a screwdriver, and see if the shaft can just be pulled out of its sealed opening. Unfortunately with these bikes, the metals used in drive shafts and motors are not always heat-treated or hardened to the standard they should be, so shearing and premature failure can happen.
4) The gearbox or something in it has blown, unfortunately for the same reason above, quality related.
Its fair to say that these great little bikes are here for a good time, but not for a long time.

I would replace the seal AND the shim also if the shim appears faulty.

remove the sprocket, hook out the seal and replace, make sure all is clean, and no grass or wire has caused it. You can usually remove the spacer that is over the shaft that the sprocket holds against, this will give room on the inside of the seal to hook it out

The oil seal on the sprocket shaft has probably gone bad. Remove the sprocket then clean the shaft to aid in removing the old seal. A screw driver and small hammer is used to pry the outer edge of the seal inward. Realize that you will butcher the old seal in the process of removal. The only other way to remove the old seal is to pull the engine and split the center case.

where is leaking oil from....behind the front sprocket??

if so your chain is TOO tight

if this is your problem, replace the output shaft seal and check drive line for abnormal wear before readjusting chain tension.

Get a new seal before taking the sprocket off. Remove the sprocket cover plate. You will see two small bolts holding a lock plate on the sprocket. Remove the bolts, lock plate and sprocket. Clean the sprocket shaft so the new seal is not damaged when installing. Now just pry out the old seal any way you can with a screwdriver and small hammer. Install the new seal flush with the casing.

Most likely the brush motor is bad and not turning, part #2036757, would sell for around $23. To get to it, start taking out screws on the top cover, that motor is under the that top access cover.