fwiw, see if using this in your .conf file makes any difference.
auth-user-pass /etc/openvpn/secret
I also observe your custom /etc/config/openvpn config file is pointing to certificate and key files which don't exist. The contents of the file may be conflicting with the .conf file. I would suggest removing entries from the file so it just reads:
config openvpn 'NordVPN'
option enabled '1'
option config '/etc/openvpn/my-vpn.conf'
Otherwise, review my v1.1 guide (for 17.01/18.06) which I know works with Nordvpn the last time I tested it.
https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=279
Other info
https://openwrt.org/docs/guide-user/services/vpn/openvpn/client-luci