I have nordvpn and have used the openvpn config files on their website for a while on gnome and it has worked perfectly. I switched to KDE and in the network manager I can import vpn settings from the config file the same as in gnome and enter my username and password, but when I press the connect button nothing happens. It doesn't give any error it just does nothing.
Make sure that you have all the required packages like
network-manager-openvpn (or in my case
network-manager-openconnect) or some other packages in
apt search networkmanager or
apt search network-manager. Unfortunately, when these packages are missing, it does not display any error message -- it just "does nothing"