Here Is (part of) a script that I am using for my game:
local sss = game:GetService("ServerScriptService")
--.........
sss:FindFirstChild("Game").Disabled = true
--.........
sss:FindFirstChild("Game").Disabled = false
It runs fine in studio, but in play mode this error pops up:
Players.PyccknnXakep.PlayerGui.GameScript:69: attempt to index a nil value
So I put in a short snippet of code inside of the Game script located in the serverscriptservice:
print(script.Parent)
It returns ServerScriptService, but the GameScript in the StarterGui won't disable it and says it doesn't exist. This might be a small syntax error, but any help would be greatly appreciated.

4 comments
could try :WaitForChild("Game")
Is "GameScript" a local script or a server-side script?
If GmaeScript is part of StarterGui, how will you get it through Server Storage?
GameScript is a localscript in the Startergui and WaitForChild returns an infinite yield and breaks the script.