Gui opening and closing?

When i am in roblox studios just testing it, it works. But in the actual game it does not. I have tried multiple ways of fixing it but failed to figure it out. Here is my code.

ButtonGuiScript = script.Parent.ButtonGuiScript
Frame = script.Parent
GUI = script.Parent.Parent
MainMenuButton = Frame.MainMenuButton
MainMenuFrame = script.Parent.Parent:FindFirstChild('MainMenuFrame')
StartScreenbutton = Frame.StartScreenButton
StartScreenFrame = script.Parent.Parent:FindFirstChild('StartScreenFrame')
Inventorybutton = Frame.InventoryButton
InventoryFrame = script.Parent.Parent:FindFirstChild('InventoryFrame')


function MainMenubuttonClicked()
	wait(0.1)
	script.Parent.Parent:FindFirstChild('MainMenuFrame').Visible = true
end

function StartScreenbuttonClicked()
	wait(0.1)
	script.Parent.Parent:FindFirstChild('StartScreenFrame').Visible = true
end

function InventorybuttonClicked()
	wait(0.1)
	script.Parent.Parent:FindFirstChild('InventoryFrame').Visible = true
end

Inventorybutton.MouseButton1Click:connect(InventorybuttonClicked)
MainMenuButton.MouseButton1Click:connect(MainMenubuttonClicked)
StartScreenbutton.MouseButton1Click:connect(StartScreenbuttonClicked)

I am getting the following errors inside roblox

• MainMenuButton is not a valid member of frame. stack begin script 'Players.jo3thebomb.PlayerGui.MainScreenGui.ButtonScreenFrame.ButtonGuiScript'. stack end

-InventoryCloseButton is not a valid member of frame. stack begin script 'Players.jo3thebomb.PlayerGui.MainScreenGui.InventoryFrame.InventoryGuiScript'. - stack end

-ShopButton is not a valid member of frame. stack begin script 'Players.jo3thebomb.PlayerGui.MainScreenGui.MainMenuFrame.MainMenuScript'. - stack end

What you didn't do was add locals. Locals are what substitutes something for something else, and this is one core thing you left out.

local ButtonGuiScript = script.Parent.ButtonGuiScript
local Frame = script.Parent
local GUI = script.Parent.Parent
local MainMenuButton = Frame.MainMenuButton
local MainMenuFrame = script.Parent.Parent:FindFirstChild('MainMenuFrame')
local StartScreenbutton = Frame.StartScreenButton
local StartScreenFrame = script.Parent.Parent:FindFirstChild('StartScreenFrame')
local Inventorybutton = Frame.InventoryButton
local InventoryFrame = script.Parent.Parent:FindFirstChild('InventoryFrame')