I want this gui to hide at the beginning and show later Does not work Why?

game.ReplicatedFirst.ScreenGui.Enabled = true

Yep just that

The script is also inside the screen GUI in replicated first

wait(10)
game.ReplicatedFirst.ScreenGui.Visible = true

Make sure the Frame/GUI starts off as not visible

game:GetService("ReplicatedFirst"):WaitForChild("ScreenGui").Enabled = false This is the best way I know of ^

WaitForChild, as it says in the name, waits until it finds a child named "ScreenGui" where you tell it to search for it, faster than just wait(10).

Also you should also use game:GetService(service) instead of game.Service, if something went weird like an exploiter renamed a service somehow it should still work because it's looking for the class name instead of the instance name of the service