I'm using Roblox's new dialogue editor, and you can make functions when a player clicks a response, which I've done. However, the code I'm using is working fine, because it goes to the position I want it to, it's just not showing up at all. I can't see it.
return function(player, dialogueFolder)
local Gui = game.StarterGui.GUI
Gui.Enabled = true
Gui.Frame:TweenPosition(UDim2.new(0.5, -200, 0.5, -250), 'Out', 'Sine', 1)
end
(First post on this website FYI.)

2 comments
Are you actually calling the function? (Considering it''s returned, so I'm assuming it's coming from either a ModuleScript or it's inside another function)
Each player has a GUI folder called the PlayerGui. That is what they see. The StarterGui is what you see in Studio, and what is replicated (copied) to all players in the server upon joining or respawning, so you would change it in the PlayerGui. If this is a LocalScript, use script.Parent.Parent etc..