Why this not Working in Online Server?

local ID = 1038094
local GUI = game.ServerStorage.GUI1

game.Players.PlayerAdded:connect(function(Plr)
Plr.ChildAdded:connect(function()
if Plr:IsInGroup(ID) and Plr:GetRankInGroup(ID) >= 4 then
GUI:Clone().Parent = Plr.PlayerGui
end
end)
end)

I see no point in using Plr.ChildAdded:connect(function() . You should be using ReplicatedStorage instead of ServerStorage So don't forget to move the GUI! Your code should look like this:

local ID = 1038094
local GUI = game.ReplicatedStorage.GUI1

game.Players.PlayerAdded:connect(function(Plr)
	if Plr:IsInGroup(ID) and Plr:GetRankInGroup(ID) >= 4 then
		GUI:Clone().Parent = Plr.PlayerGui
		end
end)