fwiw, the Nordvpn instructions mentions using the snapshot version of luci-app-openvpn package.
If you are unable to upload the OpenVPN configuration file , do the following procedure to download the snapshot version of the luci-app-openvpn plugin:
fwiw, I'm no expert, but one other thought I had is whether 'snapshot' packages can be safely used with older 'stable' releases such as 18.06.x? My understanding it is normally a bad practise to 'update' existing packages. No harm in trying I suppose.
At first glance, NordVPN's own web guide also may not have properly implemented 'kill switch' btw.
Update: Have you tried OpenWrt 19.07 snapshot, these appear to include the 'OVPN configuration file upload' option in luci-app-openvpn package.
Usually you can't mix LuCI packages from a new branch (like 19.07 or master) to firmware of an old branch like 18.06. The packages may reflect other changes in the LuCI main components or in the underlying OpenWrt.
Changing the opkg download feed definitions is usually a bad idea.
fwiw, if you are still using 18.06.x, you may want to consider using my v1.1 guide written for windows users if you are struggling with setting up openvpn client. https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=279
I am using OpenWrt 18.06. I tried to use OpenWrt 19.07, but my Wireless USB (TP-link wn7200nd) didn't work. What could I do? I want to use my Raspberry Pi as Router VPN with an external USB Wifi