How to fix the issue that my gui will only show when launching in studio?

I am working on an horror game and the part I have an problem with is the GUI for the mainmenu. It works fine in Roblox Studio but not when I play online.

Script:

script.Parent.Touched:connect(function (hit)
  local player
  if hit.Parent:FindFirstChild("Humanoid") then
      player = game.Players:WaitForChild(hit.Parent.Name)
  end
  local gui = player:WaitForChild("PlayerGui"):WaitForChild("MainmenuGui")
  game.Workspace:WaitForChild(player.Name).Humanoid.JumpPower = 0
  game.Workspace:WaitForChild(player.Name).Humanoid.WalkSpeed = 0
end)

Error: Infinite yield possible on 'Players.BeAHacker666.PlayerGui:WaitForChild("MainmenuGui")'

I found an "solution" on someone else's problem with the same issue that says I need to use localscript instead of serverscript. But then it doesn't even work in Studio.

I recommend you studying the code I added so you can understand how you can access guis in a player. I added comments all over the scripts to make you understand a little better. ^^

I'm not quite sure about what you want to do with the "MainmenuGui", but here is a way to access it:

In the part that are going to be touched add:

script.Parent.Touched:connect(function(hit) -- Brick Touched
	script.Disabled = true -- Disabling the script. (So It doesn't activate hundreds of times)
  	if hit.Parent:FindFirstChild("Humanoid") then -- Checking for humanoid
		print("It is a humanoid!") -- Printing that it found the humanoid
    	local player = game.Players:WaitForChild(hit.Parent.Name) -- Finds the player who touched
  		player.PlayerGui.ThisWorks.Disabled = false -- Enables a localscript inside the player
	end -- Closes the if statement
	wait(1) -- Waits one second before activating the touched script
	script.Disabled = false -- Script is able to execute again
end) -- Ends function

^^^^^^^^^^^^^^^^^^^^^^^^^^ So this is a server script which is inside the brick. It activates a localscript inside the player. But hey! We gotta make a localscript too! There are two ways in doing this:

1 - Making a serverscript which moves the script into the player (Will be able to change name of the script)

2 - Just adding a localscript into the PlayerGui (Can't change name of the script)

Option 1:

game.Players.PlayerAdded:Connect(function(plr) -- Fires when a player joins

	local Storage = game:GetService("ServerStorage") -- This is where the LocalScript is storaged

	local ThisWorks = Storage:WaitForChild("ThisWorks") -- This is my localscript

	ThisWorks.Parent = plr.PlayerGui -- My localscript's parent will be the player's playerGui
	
	print("Finished") -- Prints in console that the script is finished.
end)

This is how my workspace looks like: https://gyazo.com/4a29dda5e09982919fe8b039ae36fc62

I have now moved my localscript into the player, and the script will be enabled whenever I touch the part.

The last part you have to do is to put something into the localscript. I just added some random stuff:

local player = game.Players.LocalPlayer -- Finds the localplayer

print("It works? How nice!") -- Prints that the script is enabled after I touched the brick

script.Disabled = true -- Disables the script (So I can fire it over and over again)

I hope you understand what I'm trying to show you. xD I know this is kinda messy but I'm sure you'll figure it out. You can now add:

local gui = player:WaitForChild("PlayerGui")
local mainMenu = gui:WaitForChild("MainmenuGui")

And there you have it. You got access to your MainmenuGui. You can now do whatever you want with it. I hope you find this helping. ^^ Enjoy! ~WaterFoox