String Problem,can anyone help me?

First, tab and space your code properly.

game.Players.PlayerAdded:connect(function(plr)
	if player:GetRankInGroup(config.GroupId.Value) >= config.RankId.Value then 
		plr.Chatted:connect(function(msg)
			if string.lower(chat) == "sf" then
				if not game.Workspace:FindFirstChild("Thing") then
					game.Lighting.Thing:Clone().Parent = game.Workspace
				end
			elseif string.lower(chat) == "close sf" then
				if game.Workspace:FindFirstChild("Thing") then
					game.Workspace.Thing:Remove()
				end
			end
		end
	end)

You'll see you're missing a final end) and have misplaced the final parenthesis ).

You never defined player (your parameter is plr).

You never defined config.

You should test incrementally. Not write 20 lines and hope it all works. That's a stupid way to do things. Write two lines at a time, and then test. Also, use the output since it would have told you all three of these errors.