Local Script:
wait()
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Lce = ReplicatedStorage:WaitForChild("LCE")
jun = Players.LocalPlayer
jun.Chatted:Connect(function(Msg)
local msg = Msg:lower()
if string.sub(msg, 1, 7) == "LG" then
wait(0.1)
Lce:FireServer()
end
end)
Server Script:
wait()
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Lce = ReplicatedStorage:WaitForChild("LCE")
Lce.OnServerEvent:Connect(function(player)
print(player.Name , "Works")
end)

7 comments
if string.sub(msg, 1 ,2) == 'LG' then *
still nothing
Remove Msg:lower() because "LG" is capital. Or just do Msg:upper().
even with the lg not being capital it's still the same
why r u using sub? u should use string.find
Make else statements to check if the if statement's conditional is actually true(In your LocalScript, the "LG" part of the if statement on line 8 should be lowercase because you use the lower function and that makes all the string's characters lowercase. In addition, you are checking for 2 letters when you are checking for 7).
if Msg:sub(1, 2):lower() == "lg" then ... end