sudo: ifdown: command not found

The error message "sudo: ifdown: command not found" suggests that the "ifdown" command is not installed on your system or is not in the system's path. The "ifdown" command is typically used to bring down a network interface.

To resolve this issue, you can try installing the "ifupdown" package which contains the "ifdown" command. You can use the following command on Ubuntu/Debian:

sudo apt-get install ifupdown

Alternatively, you can try using the "ip" command to bring down the network interface. For example:

sudo ip link set <interface> down

Replace `<interface>` with the name of the network interface that you want to bring down.