This could be caused by a battery that has failed or a problem with the charger port or battery charger. It would take some testing to figure out which part is causing the problem.
When the scooter is on the ground and you pull the throttle and it start then stops the battery gauge should be dropping down near low which indicates that the battery pack is dropping Voltage under load. This is most commonly due to a faulty battery in the battery pack, however it could also be because a problem with the charger or charger port. If the charger light turns green for fully charged then usually the charger is working. If the charger light does not turn red for a moment after it is plugged in before it turns green then there is a good chance that the charger port or plug on the charger is not working. If the scooter was in our shop I would perform the following tests to determine the problem.
If you do not have access to a multimeter to test the battery charger and charger port with then having the batteries load tested is the first thing that I would do.