This is a good place to start from. If these solutions do not solve the problem please let me know and we will take it from there.
New parts such as a battery charger, charger port, or battery pack could be thrown at the scooter to see if that helps, or the battery charger, charger port, and battery pack could be tested to see which part is causing the problem.
To go the testing route I would test the battery charger to see if it has Voltage at its output plug. If the charger does not have any Voltage output then it needs to be replaced. Then I would test for Voltage at the charger port, this test will determine if charger port and wiring harness are working, and how much Voltage is in the battery pack. A 24 Volt battery pack should have around 26-28 Volts in it. If its battery pack's Voltage is under 23 Volts then it may be worn out or defective and need to be replaced.
By testing the Voltage of the battery charger and charger port the part which needs to be replaced can usually be determined.