The spark when plugging the battery pack into the scooter's controller or wiring harness is normal and nothing to worry about. Virtually all electric scooter battery packs do that when they are plugged into the scooter. I have heard that the spark is from the controller's capacitors initially charging when the battery pack is first plugged in.
The first step I like to take for troubleshooting is to unplug the brake lever switch connector from the controller to make sure that a faulty brake switch is not causing the problem. If the scooter runs with the brake switch disconnected then the brake switch is locked into the on position and will need to be replaced.
The next step I would take is to unplug the power input connector from the controller and with the power or key switch on test for Voltage at that connector. If it has power then the power or key switch and fuse or circuit breaker are working so they can be ruled out as being the problem. If it does not have power then the power or key switch and fuse or circuit breaker will need to be tested to see which part is not working.
If that does not solve the problem the next thing I would do is test the battery pack Voltage with the power off, and again with the power on and throttle fully engaged. Since the battery pack is fully charged it will be several Volts above its rating already and its Voltage should not drop more than around one Volt with the power on and throttle engaged. If the battery pack Voltage is significantly dropping under load then one or more of the batteries in the pack is faulty and the battery pack will need to be replaced.
Controllers have low-Voltage cutoff circuits built into them which turn them off when the battery pack Voltage reaches a certain level. These low-Voltage cutoff levels are usually as follows:
Please let us know how it goes.