The little screw in the little sprocket does not grip well on the axle of the extruder motor. So I looked it up in the building manual, and on the picture they actually have a ribbed axle, but I only received smooth ones.
When the sprocket doesn’t turn, does the sprocket kind of stutter? If so it can be a problem with the gears being too close - the small gear might not have leverage to turn the large one.
If you help the large gear along by hand, does the extruder motor turn? If so, move the motor sprocket further away from the feeder sprocket.