How to open a GUI from brick?

There are few cases where you'll need to manipulate GUI's within StarterGui. The StarterGui's purpose is to replicate GUIs to the PlayerGui, which is where all the UI is stored that a user can see.

Your code has three main issues:

1) You are attempting to edit a GUI within StarterGui, not PlayerGui

2) You never find the Player who touched so you may access PlayerGui

3) Assuming this is a Script, you cannot access PlayerGui without using a RemoteEvent or RemoteFunction

The below fixes are based on the above issues. You will need to add a RemoteEvent to ReplicatedStorage and name it "EnableGui" and add a localscript to PlayerGui inside of the GUI you are enabling.

-Script

local EnableGui = game.ReplicatedStorage:WaitForChild("EnableGui")
function Touch(hit)
    if hit.Parent:FindFirstChild("Humanoid") then
        EnableGui:FireClient(game.Players:GetPlayerFromCharacter(hit.Parent))
    end
end

script.Parent.Touched:Connect(Touch)

-localscript

local EnableGui = game.ReplicatedStorage:WaitForChild("EnableGui")

EnableGui.OnClientEvent:Connect(function()
    script.Parent.Enabled = true    
end)

If you have any issues or questions, then feel free to comment below.

I don't know if this helps but try using this:

script.Parent.Touched:Connect(function(hit)

print("Debug Message")
local playername = hit.Parent.Name
local player = game.Players:WaitForChild(playername)
local playergui = player:WaitForChild("PlayerGui")
local GUI = playergui:FindFirstChild("Test")


if GUI then
    GUI.Enabled = true
end

end)

I did test it but you know Roblox studio, one thing might work for someone but not for another. Anyways I hope this helps! :D

P.S. I don't know why it didn't put the whole code block in but eh, works just the same