lock

I'm not sure what the problem is here. When holding the clutch lever the chain spins freely as it should, but when I let it go to start the engine, it completely locks and I can't pedal or spin it at all. The engine is brand new, I've never started it. This is my first, so any help is appreciated.

What is the size of the little screw that locks the flower nut in place? I need to pick up a new one at a hardware store, or order one online.

I lost mine, need to buy some extras, what site sells them and what are they called? Or what size is it so I can pick one up at a hardware store?