Why are you trying to use game.Players.LocalPlayer on a “server script”? Don’t you know that you can’t use Players.LocalPlayer on a server script? Here is the fixed code:
game:GetService("ReplicatedStorage").MissaoEvento2.OnServerEvent:Connect(function(player)
local TweenService = game:GetService("TweenService")
local missionScreenGui = player.PlayerGui:WaitForChild("ObjetivoGUI") -- SHOULD be "player" not "game.Players.LocalPlayer".
missionScreenGui.Enabled = false
end)