If you have that script in StarterGui, it won’t work
Put it in StarterPlayerScripts, and you can also remove game.Players.PlayerAdded:Connect(function()
For GUI, you can do what @LxckyDev said. For the “play” button, you can add an event listener in the same StarterPlayerScript as mentioned before, like so:
local playBtn = game.Players.LocalPlayer.PlayerGui -- finish the path
playBtn.MouseButton1Click:Connect(function ()
game.Lighting.Blur.Size = game.Lighting.Blur.Size - 3
-- or tween it
end)