Diagnosis
The controller is receiving throttle signal but cannot deliver power. The click is the main relay. Primary cause is a loose motor phase wire connection.
How to Fix It
Power off the scooter. Tip it on its side. Locate the three thick yellow wires running from the deck to the rear wheel. Trace them to the connector near the wheel. Firmly press this 3-pin bullet connector together until it clicks. Ensure the rubber boot is seated.
If That Didn't Work
The controller's MOSFETs may have failed due to water ingress or overload. This requires a controller replacement, part number SM-SW-LT2.
Heads Up
Do not repeatedly press the throttle while hearing the click. You can overheat the relay and cause further damage.