New to Nix

I have never installed a nix package on Steam Deck under SteamOS before. I followed his instructions from this website , everything is ok without problem, then I installed an app with this command nix-env -iA nixpkgs.mullvad-vpn. When I finish installing an app, I open Mullvad GUI, I see a message

    
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 fix this issue

My Nix version is 2.32.4 and my SteamOS version is 3.7.13