How can I access playergui through this script?

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        script.Haxxed:Clone().Parent = character.PlayerGui
    end)
end)

This doesn't work for some reason!

Well, I see your issue. Here's your script fixed:

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        script.Haxxed:Clone().Parent = player.PlayerGui -- The PlayerGui is inside the Player not Character.
    end)
end)