This works in the studio but when I try it in game, it doesn't work.
local pl = game.Players.LocalPlayer:FindFirstChild("PlayerGui")
pl.IntroGui.Value.Value = 1
I tried using LocalScript, it didn't work at all.
I think game.Players.LocalPlayer is the mistake but I can't figure it out.

6 comments
A LocalScript is necessary, since the server can't access the clients' PlayerGuis' descedants - only when the server parented it. You should also know that everything wouldn't load in immediately. Show the hierarchy of the gui
I know that, that's why I'm checking with you. Should I use 'WaitForChild("PlayerGui")'?
Have you tried using this as a local script, inside the GUI?
I need the script to be in workspace.
where is the number value located?
The value is located in the Gui and the script in workspace.