You can find them in the user manual, on the scooter or in the NIU APP after you've bound the scooter.
SN = serial number
VIN = frame number
In the user manual
You will find a QR Code that holds the SN number of your scooter.
At the top of the QR code, you can find the SN number. At the bottom of the QR code, you can find the VIN number.
On the scooter
You can find a sticker with the SN number underneath the seating part.
The VIN number is engraved in the frame of the scooter, also under the seating part.
In the NIU APP
In the tab 'Me', click on your device. You'll find SN and VIN number there.