Help with Visible Gui?

The problem is that you're trying to get the Player's Gui from the StarterGui when the Player's Gui is inside the PlayerGui which is inside the Player. To get the Player from the Touched event we will need to use GetPlayerFromCharacter().

Debounce = false
script.Parent.Touched:connect(function(Hit)--The Function listen for the Touched Event.
   local Player = game.Players:GetPlayerFromCharacter(Hit.Parent)--Gets the Player From the
    if not Debounce and Player then
    Debounce = true
     Character.
    local Gui = Player.PlayerGui.TouchedGui.Frame--Variable for Quick access.
    Gui.Visible = not  Gui.Visible--If Visible is False it turns it true,If Visible is true it turns it false.
    wait(2)
    Debounce = false
    end
end)

To fix your problem, you need to access the PlayerGuirather than the StarterGui. To get the player, we'll have to get the character from the part that touched it, then go from there. (Also you forgot to connect the function)

script.Parent.Touched:connect(function(Part)
    local Player = game.Players:GetPlayerFromCharacter(Part.Parent) --Function that gets the player from the character, which is that parts parent.
    if Player then --Makes sure Player exists
        local Gui = Player.PlayerGui.TouchedGui.Frame
        if Gui.Visible == false then
            Gui.Visible = true
            print'True'
        else
            Gui.Visible = false
            print'False'
        end
    end
end)

Ok, now your script should work. If you have any further problems/questions, please leave a comment below, and I'll see what I can do. Hope I helped :P

-Dyler3