I haven't made a bike from mobility scooter parts, but I do regularly repair mobility scooters. The error code (led flashes) probably depend on the manufacturer, have you used the motor/speed controller from the mobility scooter? Assuming power is OK - They don't like it when the throttle isn't exactly in the centre, (throttle must be in the neutral position when the controller is turned on) that's the first thing I'd check.