when i start playing with test the output showing me this !

========= 20:06:42.175 - ScreenGui is not a valid member of PlayerGui

and of course i have a ScreenGui into my PlayerGui

Location : ServerScriptStorage w/ script

And it strange because yesterday it was working really fine :/

    for i,v in pairs(game.Players:GetPlayers()) do
        v.PlayerGui.GUI.Block.Visible = true
    end

if someone know why i will be great to help me : )

Well, in the server's perspective, a Player's decedents aren't there, so that is why you are getting the error.

Also, if you want to make a GUI invisible once the server starts up, you could always untick the Visible property of that GUI so it does not appear, instead of doing it when the player joins.