webmin: Failed to save interface : Missing or invalid interface name

I am attempting to use webmin to set up a firewall/gateway router according to this set of instructions. The network interface (enp6s0) that is connected to the internet is listed in both the “Active Now” and the “Activated at Boot” tabs. When I click on the network interface I am given the opportunity to change its parameters. The network interface (enp4s7) that will receive the shared network connection is only listed under the “Active Now” tab with a Status of “Down”. When I click on this interface the check box next to it is toggled and I can not see a way to edit its setting like I can for the other network interface. I have attempted to use the “Add a new interface” button using the name listed by webmin on the previous page (enp4s7) but I get the following error:

Failed to save interface : Missing or invalid interface name

I can toggle the Status of this network interface with “ifconfig enp4s7 up” and “ifconfig enp4s7 down” but this doesn’t seem to help. The contents of my /etc/network/interfaces file looks like this:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto enp6s0
iface enp6s0 inet dhcp

I have attempted to make changes to this file to include the enp4s7 interface that I expect will allow me to edit it in webmin but this only results in me no longer being able to access webmin until I restore the file back to its original state. I would really appreciate any help with this that anyone can offer. Thank you in advance.