Can Somone Tell Me Why This Does Not Work?

You've done almost everything wrong. I will re-write your script with some explanations:

local prefix = "!"

function playerMsg(plr,mesg,sm) -- sets function, plr being the sender, mesg being the message, and sm being whether it's a system message
	local players = game.Players:GetChildren() -- gets all the players
	for _,v in pairs (players) do -- does the following for every player
			local find = v.StarterGui:FindFirstChild("GUI_NAME") -- finds the gui
			if find then --  if the gui exists
				if sm == false then
					find.Announce.Frame.MSG.Text = mesg -- changes the text
					find.Announce.Frame.Type.Text  = "Message" -- changes the type
					find.Announce.Frame.By.Text = plr.Name -- sets the text to sender's name
				elseif sm == true then
					find.Announce.Frame.MSG.Text = mesg
					find.Announce.Frame.Type.Text  = "System Message"
					find.Announce.Frame.By.Text = "System Message"
				end
		end
	end
end

game.Players.PlayerAdded:connect(unction(p) -- when a player joins
	p.Chatted:connect(function(msg,recipient) -- when a player talks, msg is the text, and recipient is whoever the msg is whispered to (nil if not a whisper)
		if msg:sub(1,prefix:len()+3) == prefix.. "m " and p:GetRankInGroup(2679030) >= 9 then -- checks to see if the first part is the prefix and 'm'
			print(p.Name.. " sent a message!") -- prints
			playerMsg(p,msg:sub(prefix:len() + 3),false) -- calls function
		elseif msg:sub(1,prefix:len()+4) == prefix.. "sm " and p:GetRankInGroup(2679030) >= 9 then -- you only need one 'then'
			playerMsg(p,msg:sub(prefix:len() + 4),true)
		end
	end)
end