Do this if you want to fix ProtonVPN installation failure in Parrot OS.
- Download latest stable ProtonVPN .deb file (This is link: https://protonvpn.com/download/protonvpn-stable-release_1.0.1-1_all.deb).
- Double-click on it and install it quickly (Use package manager of Parrot OS).
- Go to command console.
- Type “sudo apt-get update && sudo apt-get install protonvpn” in it and press enter.
Now it should install ProtonVPN without any problem.