Make sure /etc/network/interfaces looks like this.
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d
(The comments starting with # are actually not important. You can skip
them if you wish.)
Then make sure /etc/network/interfaces.d/30_non-qubes-whonix looks
like this:
whonix-gw-network-conf/30_non-qubes-whonix at master · Whonix/whonix-gw-network-conf · GitHub