No CAN network when running a query or flash attempt
If you run a python3 ~/katapult/scripts/flashtool.py -i can0 -q or ~/klippy-env/bin/python ~/klipper/scripts/canbus_query.py can0 or are trying to flash a device with a command like python3 ~/katapult/scripts/flashtool.py -i can0 -u b6d9de35f24f -f ~/klipper/out/klipper.bin but you are seeing an error along the lines of “unable to bind socket to can0” or “failed to transmit, network is down” then your can0 “interface” on your Pi isn’t running.
First thing to check is your /etc/network/interfaces.d/can0 file. Make sure it exists and you have no typos in it.
Seperate USB-CAN adapter (U2C/UTOC/etc.)
If you are using a seprate USB to CAN adapter (U2C/UTOC/etc.) then double check that the USB cable connecting the devices is plugged in and not loose. If you never get a response to a query (ie. the can0 interface has never shown at all) then you may have a dodgy USB cable. I have personally seen a handful of usb-c cables that don’t actually have the data pins hooked up (they are power only). If the adapter doesn’t show to an lsusb then your cable is probably dodgy.
If it shows up to an lsusb but an ip link show can0 shows “Device can0 does not exist” then you might have a bad firmware or something on your device. Reflash it with the appropriate firmware either from a manufacturer github repository or other source (like candlelight). If there are instructions back on the voron_canbus/can_adapter folder then follow those.
USB-CAN-Bridge mode mainboard
Check that the Pi-to-mainboard USB cable hasn’t come loose or anything, and that the mainboard is actually powered up. An lsusb should show the mainboard up as a can adapter device. If it’s not showing as a can adapter device then do an ls /dev/serial/by-id. If you see your mainboard there then either it’s still in Katapult mode, or you haven’t flashed usb-can-bridge klipper to it (or the flash didn’t take). If that is the case then reflash your mainboard as per the voron_canbus/mainboard_flashing instructions and take extra care that the klipper make menuconfig settings are 100% correct for your board.