Probably your clutch is dragging.
When you "disengage" the clutch, it is still rubbing against the pucks. It will still run if your are still pedaling at speed, but as soon as wheel speed drops too far below engine speed, the friction bogs down the engine until it dies.
In a nutshell, with the bike stopped and idling there is too much drag from the clutch for the engine to run. The clutch is not disengaging far enough.
If you keep the throttle open when stopped (or going slow), you give the engine enough fuel to overcome the drag and it will still run. Close the throttle and the drag is too much. The engine dies.
If you are interested in knowing how to adjust the clutch post back here, I would be happy to write up a guide.
Yes how do i adjust the clutch? I appricate the help