You may need to install this special library for that to work: GitHub - ProtonVPN/protonvpn-nm-lib
The Arch-equivalent of this set of instructions is here: ProtonVPN - ArchWiki
Another worthwhile consideration would be to learn how to use the CLI version of the app: ProtonVPN - ArchWiki
For a VPN, learning to use the CLI version is generally not that bad because there are probably less than half a dozen commands you will ever use. Connect, set the killswitch on, change servers, set the killswitch off, disconnect…that’s basically it unless you need to whitelist IPs or other weird stuff.
Who knows, you might even turn into a weirdo like me and grow to prefer the CLI version of the app. 