Game Menu works in Studio but doesn't on a game client?

I've ran into this issue numerous times when I make something that involves using PlayerAdded. The script is inside a GUI, and is a LocalScript.

local Players = game:GetService("Players")

function onPlayerAdded(player)
    local mainMenu = script.Parent
    mainMenu.Name = "Main Menu"
    local frame = mainMenu:WaitForChild("Frame")
    frame.Parent = mainMenu
    frame.Visible = true
end

Players.PlayerAdded:connect(onPlayerAdded)

for _,player in pairs(Players:GetPlayers()) do
     onPlayerAdded(player)
end

--Plays menu music
local MenuMusic= script.Parent.MenuMusic
MenuMusic:Play()

EDIT: I fixed it by replacing various lines with others.