LocalPlayer is a local argument(i think) than can only be done on the client to get the play itself. ServerScriptService doesnt have a local player because it is the server. If you want to change the players gui, you can either access it from the game perspective.
local plrGUi = game.Players.Player1.PlayerGUI
--or
for i,v in pairs(Players:GetPlayers()) do
local plrGUi = v.PlayerGUI
end
or you can use remote events to fire the client for the client to change itself.