Pamboo

Do you mean that this command nix-env -iA nixpkgs is not recommended ? For what reason ? If it's not recommended, how do I uninstall an app that I installed packaged with nix-env ? Then I add a configuration.nix with this command kate /etc/nixos/configuration.nix , then I copy and paste

  environment.systemPackages = [
    pkgs.mullvad-vpn
  ];

is it correct ? Because I don't have a "nixos" folder in my /etc/. After creating a .nix configuration for a Mullvad package, how do I install this app ? I have already executed this command nix-shell -p mullvad-vpn before but it gives a result

Error: Management RPC server or client error  

Caused by:  
         0:   transport error  
         1:   No such file or directory (os error 2)  
         2:   No such file or directory (os error 2)  

I found the mullvad-daemon , nix-daemon files in /nix/store/svf..............-user-environment/bin/mullvad , I don't know that's it. I copied and pasted both files into ~/.config/systemd/user/ then I open the terminal from ~/.config/systemd/user/ , I typed systemctl --user enable mullvad-daemon.service, it says : Failed to enable unit: Unit mullvad-daemon.service does not exist

nix and mullvad vpn are installed on my steam deck under steamos 3.7 now but it's the same issue as before (Management RPC server or client error). Also I can open the Mullvad VPN GUI application which works but there is a message on the app :

Unable to contact the Mullvad system 
service, your connection might be unsecure. 
Please troubleshoot or send a problem 
report by clicking the "Learn more" button.

I don't know how to solve this issue

I followed his instructions up to step 3 but when I typed nix --version , it says bash: nix : command not found . Did I make a mistake ?

I think I will opt for the second solution. Nix is ​​not installed under SteamOS 3.7, how to correctly install nix with mullvad vpn without having issues ?

Yes, I ran the A.B.T. script by denis-g but did not install Cryo Utilities

Explain to me why I should do a ‘revert’ by A.B.T in the Medium post if I want to install SDWeak ?

What is "ananicy-cpp" ?

Now, I'm hesitating to remove all the scripts from A.B.T because it's not the same as Cryo Utilities and SDWeak. I found out how to remove the scripts in a blog by A.B.T. Why did I want to remove the scripts from A.B.T.? Because I saw the instructions from SDWeak saying "Before installation, be sure to remove Cryo Utilities if it was previously installed.". Will it be the same case as the A.B.T. scripts? Will there be any issues if I install SDWeak?

I didn't understand what you mean. This command allows to install on Steam Deck under SteamOS to improve SD performance, I used this guide in the past. But I don't know how to uninstall it, that's what I want

I don't think it will be possible because I have never made a backup on my Steam Deck

I'm really amazed that it works so well. Thanks everybody