Same here with my Audi A5:
Started the link with "sudo ip link set can0 up type can bitrate 500000" and then started cansniffer, but nothing.
This page:
https://jazdw.net/tp20 suggest that you have to open a channel by sending commands to a control unit.
But that also means that with VW/Audi/Skoda it is not possible to sniff on the gateway can. You can only talk to a specific device.