You need to define the debounce above the function scope.
local debounce = false
local function mClick()
debounce = not debounce
if not debounce then
SoundS.Sound.Playing = false
Music.BorderColor3 = Color3.fromRGB(239, 155, 156)
elseif debounce then
SoundS.Sound.Playing = true
Music.BorderColor3 = Color3.fromRGB(184, 239, 160)
end
end
Music.MouseButton1Click:Connect(mClick)