When using QEMU server configured by Proxmox, I received an error: "unable to find a serial interface" after running qm terminal.

According to Proxmox Wiki, to fix this you need to specify a serial port. This can be done by:

  1. Navigating to your VM's configuration located in /etc/pve/qemu-server/[vmid].conf
  2. Appending serial0: socket on a new line to the file
  3. Restarting the VM