Gear issues with electric bikes are very common. There is a number of factors which can cause gear issues, however the fix is often relatively simple.
Maintenance:
Pulling the gear lever moves a cable back and forth which allows the gear to change. Gear cables can often cause issues. The inner cable should be able to move freely inside the outer cable with no resistance. Occasionally the plastic coating on the inside of an inner cable breaks down over time causing friction not allowing the cable to move freely and change gear. This issue can also be caused due to dirt or grease build up inside the outer cabling or, corrosion/frayed inner cabling. Regularly check the cables are clean and move freely, if not they may need to be replaced or at the very least cleaned with a wipe and fresh lubricant.
Damage:
It is rare for a shifter to fail unless very old. It is often easy to spot a failed shifter as it no longer clicks into gear, however seized cables can give the appearance that the shifter is actually broken when it isn't. It is however common to bend/break the rear derailleur leaning the bike against a wall or dropping it on the floor where the rear derailleur hits the surface can cause the hanger to bend out of shape. If your bike is not changing gear ensure that the hanger has not been bent and that the rear derailleur is sitting straight.
Setting gears:
Assuming cables are all fine and nothing is bent or worn, the only remaining cause of incorrect gearing is incorrect setting.
To reset the gear tension, place the bike in the smallest gear at the rear (7th or 8th usually) undo the gear cable clamp. The rear derailleur has 2 limit screws one screw controls the maximum limit of movement towards the wheel, the other the maximum movement away from the wheel. Adjust the limit screws so that the rear derailleur can freely move down to the bottom gear, in line with that gear, and towards the wheel, slightly over the center of the gear. Be sure to push the derailleur into position in the first gear and ensure the derailleur does not go as far as touching the spokes of the wheel or allowing the chain to fall off in between the freewheel and spokes.
With the limit screws set correctly, next is cable tension. Pull firmly on the cable and clamp it back down with no free play on the cable. Try each gear on the bike, if the bike does not go up the freewheel into the easier gears freely, the cable needs tightening slightly. If the gears do not go down towards the hardest gear freely the cable is too tight and needs to be loosened.