i had the same problem i ended up puttin two bolts on the axle on the left side where your sprocket is in between the frame and the coaster brake, then mounted the tire on making sure it was straight, then i bolted the wheel down on the frame, and saw the chain was straight, but to be honest idk if what i did was good, i think it causes tire wear, so maybe you want to try to tightening the chain a bit more