Bike chain slips off sprocket as I pedal during repairs. Any idea why? I am having an issue with a 26 inch Beach Cruiser.

Everything was put together by a bike shop however one of the chains slipped off the sprocket during transport home. When I went to put it back on, which should have been easy (flip the bike on its handle bars and reattach the chain by taking off the back wheel and pulling it backward further into the rear dropouts to apply tension to the chain). 

As a precaution I test pedaled it while it was still upside down the chain still slipped off.

I'm pretty sure the bike kit came with a second chain tensioner and for some reason I don't have it, probably due to bike shop error.   If anyone can confirm that these kits come with two, I'd appreciate it. I hate to think I'm missing a part when I'm actually not.

Side note: The bike is also motorized, I don't know if that was a needed piece of information or not. 

My question is, how do I get the chain to stay on when I pedal?
Figured out the issue. The cotter pin behind the sprocket needed to be removed and the sprocket moved backwards.

Edit: that did not fix the issue. I'm still working on it.