ScreenGui not showing up?

Well what you're doing here is your parenting the screenGui to game.StarterGui.

game.StarterGui only runs once every time the character respawns, so you wont see the changes unless your character respawns. Instead you should detect the player that is touching the part and insert the GUI into their playerGui which is what loads inside of the player instance that holds everything locally on their end. Everything from StarterGui is placed into there when they respawn.

Also you're sound ID needs to be in the correct format: http://www.roblox.com/asset?id=IDHERE

And you need to play the sound with the object's method: soundobject:Play()

I made some changes here that I documented for you so you know what it is doing:

local debounce = false; -- created a debounce so it does not trigger multiple times

function onTouched(part) -- the parameter returned in a touched event is the part that touches
	if game.Players:GetPlayerFromCharacter(part.Parent) and not debounce then -- We check to see if the part that touched can be used to obtain a character from the model. Using GetPlayerFromCharacter we can just ask if part.Parent which can be a character model can obtain a player.

		debounce = true; -- change debounce so it doesn't trigger again while running
		local player = game.Players:GetPlayerFromCharacter(part.Parent); -- we set our player with the same method I just explained above
		
		skull = Instance.new("ScreenGui")
		skull.Parent = player.PlayerGui -- Place in their PlayerGui

		tb = Instance.new("TextButton")
		tb.Size = UDim2.new(0, 1920, 0, 843)
		tb.Parent = skull

		image = Instance.new("ImageButton")
		image.Image = "http://www.roblox.com/asset/?id=24064946"
		image.Parent = tb
		image.Size = UDim2.new(0, 1920, 0, 843)

		scream = Instance.new("Sound", player.PlayerGui) --Place audio's in the playerGui instead of workspace so the sound is local to the player and can be heard fully by the player as they move forward
		scream.SoundId = "http://www.roblox.com/asset?id=266075228"
		scream:Play();
		
		coroutine.resume(coroutine.create(function() -- here I create a coroutine that will run along side the rest of your code, that way you don't have to wait an additional 2 seconds. waiting for the audio to end.
			repeat wait(1) until scream.TimePosition == 0 -- we create a repeat loop to wait every second and to break the loop when the audio's TimePosition is reset to 0 which means it would have stopped because the sound had already started from above in very fast miliseconds. 
			scream:Destroy(); -- when it hits 0 meaning it stopped, we destroy it also as you do the screenGui so we don't stack them up 
		end))

		wait(2)
		skull:Destroy()
		debounce = false; -- reset the debounce. 
	end
end

script.Parent.Touched:connect(onTouched)