It would help if you would add a hint on what the actual issue is currently. First it is an issue on how you use su or sudo, then later it's about name resolution.
Seems this could be the issue, from the log from the other person that commented:
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resolvconf: Error: Command not recognized
Usage: resolvconf (-d IFACE|-a IFACE|-u|--enable-updates|--disable-updates|--updates-are-enabled)
This might be the cause that you can reach adresses/sites using their IP adresses over the VPN, but there's no name resolution if your resolv.conf is not being updated properly.
If protonvpn requires
resolvconf, you might need to install that package, because it's not installed by default.