Why wont some scripts work in Multiplayer/Normal Mode but will in Studio/Build Mode?

Here's my local script located in StarterGui > ScreenGUI > Music (button) > [Located here]

local button = script.Parent --colours (easier to use) Grey = button.BackgroundColor3 Green = Color3.new(0,1,0) Red = Color3.new(1,0,0) White = Color3.new(1,1,1) Lightblue = Color3.new(0,1,1)

--size (pointless, was meant to be removed but didn't)

NormalSize = UDim2.new(0, 0, 0, 40) NormalPosition = UDim2.new(0, 212, 0, 0)

--sounds (This is where I get the music from so the script can activate it S1 = script.Parent.Parent.Sound1 S2 = script.Parent.Parent.Sound2 S3 = script.Parent.Parent.Sound3 ERRORSOUND = script.Parent.Parent.ERROR OPEN = script.Parent.Parent.OPEN Main1 = script.Parent.ThemeTune

--rules (eh) HasPressedPlay = false TABOPEN = false OPEN2 = false

--children (none)

function MouseIsIn()

    if TABOPEN == false then

    button.BackgroundTransparency = 0.1
    button.BorderColor3 = Green
    button.TextColor3 = Green
    button.Text = "-MUSIC-"
    S3:play()

    end

end

function MouseIsOut()

    if TABOPEN == false then

    button.BackgroundTransparency = 0.05
    button.BorderColor3 = Grey
    button.TextColor3 = White
    button.Text = "MUSIC"
end

end

function MouseIsDown()

if OPEN2 == false then

TABOPEN = true button.BackgroundTransparency = 0.1 button.BorderColor3 = Green button.TextColor3 = Green button.Text = "GRANTED"

    wait(.5)


    Main1:play()

    button.BorderColor3 = Lightblue
    button.TextColor3 = Lightblue
    button.Text = "PLAYING" 

OPEN2 = true

elseif OPEN2 == true then

button.BackgroundTransparency = 0.1
    button.BorderColor3 = Green
    button.TextColor3 = Green
    button.Text = "STOPPING"

    wait(.5)



    button.BorderColor3 = Grey
    button.TextColor3 = White
    button.Text = "MUSIC"   
    Main1:stop()
    TABOPEN = false    

    OPEN2 = false

end

end

button.MouseEnter:connect(MouseIsIn) button.MouseLeave:connect(MouseIsOut) button.MouseButton1Down:connect(MouseIsDown)player = game.Players.LocalPlayer

If you could please explain and point out any mistake I made please that would be a lot of help I place this script in a button GUI called Music with a sound file called ThemeTune and a local script (This one right here)