If no device is found after running ls -l /dev/serial/by-id reboot the zigbee2mqtt lxc and try again.
Make sure Proxmox sees the device by running ls -l /dev/serial/by-id from the Proxmox web shell.
If Proxmox sees your device, you can try setting autodev by running the below script from the Proxmox web shell.
bash -c "$(wget -qLO - https://raw.githubusercontent.com/tteck/Proxmox/main/autodev.sh)" -s 100
100 to your LXC ID.
Note: The changes will apply after a reboot of the LXC