TransitionScript(Roblox Lobby)

local TweenService = game:GetService("TweenService")
local player = game.Players.LocalPlayer
local TransitionEvent = game.ReplicatedStorage.TransitionEvent
local MainGui = player.PlayerGui:waitForChild("MainGui")
local Transition = MainGui.Transition

TransitionEvent.OnClientEvent:Connect(function(timer)
    local goal_start = {}
    goal_start.BackgroundTransparency = 0
    local tweenInfo = TweenInfo.new(2) -- add N time
    local tween_start = TweenService:Create(Transition, tweenInfo, goal_start)
    tween_start:Play()
end)