Klipper relies on a stable and permanent connection between the board and the RPi / SBC. Of course printing from SD is possible but is not following the Klipper philosophy as it is much more efficient to print from the RPi. This is called virtual_sdcard and the modern webinterfaces rely on it.
What does ls /dev/serial/by-id/* return? If nothing, then check if Debian Bullseye Bug causing Klipper to no longer find the printer board - #8 is applicable.