OpenVPN Manager does not handle usb ethernet dongle device?

Trying to add a vpn connection on my laptop, I realize that the save button is always inactive.
The same operation on my workstation goes fine.

The only difference is the ethernet hardware: my laptop doesn't have one ethernet device, so to overcome this drawback I use a USB adapter ->

[ 6.343869] ax88179_178a 2-1:1.0 eth0: register 'ax88179_178a' at usb-0000:00:14.0-1, ASIX AX88179 USB 3.0 Gigabit Ethernet, 00:0e:c6:a1:72:e7
[ 6.471805] ax88179_178a 2-1:1.0 enx000ec6a172e7: renamed from eth0

each OS are Ubuntu 18.04, Gnome shell interface.

I'm using network-manager-openvpn-gnome Version: 1.8.2-1 and GNOME Shell 3.28.4.

$ nmcli device status

DEVICE TYPE STATE CONNECTION
08:3D:88:C1:D4:D3 bt disconnesso --
08:EE:8B:FA:9B:A6 bt disconnesso --
wlp2s0 wifi non disponibile --
enx000ec6a172e7 ethernet non gestito --
lo loopback non gestito --

cat /etc/network/interfaces ->

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

#auto dsl-provider
#iface dsl-provider inet ppp
#pre-up /bin/ip link set enx000ec6a172e7 up # line maintained by pppoeconf
#provider dsl-provider

auto enx000ec6a172e7
iface enx000ec6a172e7 inet dhcp

Thanks