If you are actually on Manjaro, this simply cannot be …
Sure it can, I promptly and gleefully remove pamac after install.
But good catch in this case.
(Now does this mean OP will consult their own distro, or opt to install Manjaro? )
Proton could care less about GNU / Linux let alone Arch or Manjaro. They can’t even be bothered to test with Arch. If they did, they would know that Pamac is not available in the Arch repos. They no longer maintain the Proton VPN packages in the AUR as of the new 4.0 client.
Please contact Proton VPN support. This has nothing to do with Manjaro and it’s obvious you’re not using Manjaro.