When the Gokart Kit starts moving forward by itself, this indicates that the Ninebot S has not been switched to Gokart Mode.
Please turn off the device and follow the instructions below to enable Gokart Mode:
1. In the upper right corner of the app, tap Settings Firmware Update.
2. On the Ninebot settings page, tap "Gokart mode"
3. Enable "Gokart mode"