What is wrong with my script? bad argument #2 to 'sub' (number expected, got no value)

function guicreate(Player, Text)
	local sg = Instance.new("ScreenGui")
	local fr = Instance.new("Frame")
	local text = Instance.new("TextLabel")
	sg.Parent = game.Players[Player].PlayerGui
	fr.Size = UDim2.new(1, 0,0, 100)
	fr.Position = UDim2.new(0, 0,-1.417, 0)
	fr.Parent = sg
	text.Size = UDim2.new(1, 0,1, 0)
	text.Position = UDim2.new(-2, 0,0, 0)
	text.Parent = fr
	fr.Style = "DropShadow"
	text.BackgroundTransparency = "1"
	text.TextScaled = true
	text.TextColor3 = Color3.new(255, 248, 248)
	text.TextStrokeTransparency = "0"
	text.Text = Text
	fr:TweenPosition(UDim2.new(0, 0,0.417, 0), "Out", "Quad", 3)
	wait(1)
	text:TweenPosition(UDim2.new(0, 0,0, 0), "Out", "Quad", 3)
	wait(2)
	text:TweenPosition(UDim2.new(-2, 0,0, 0), "Out", "Quad", 3)
	wait(3)
	fr:TweenPosition(UDim2.new(0, 0,-2.417, 0), "Out", "Quad", 3)
	wait(2)
	sg:Destroy()
end

local Adminlist = "Kwok6140", "Player1", "Player2"


game.Players.PlayerAdded:connect(function(player)
	player.CharacterAdded:connect(function(character)
		guicreate(player.Name, "Welcome to this Airport ,"..player.Name)
		player.Chatted:connect(function(msg)
			print(string.sub(string.lower(msg),1,6))
			print(":kill")
			print(string.sub(string.lower(msg),1,6) == "kill")
			if string.lower(msg) == ":kill" or  string.lower(msg) == ":kill me" then
				print("Kill")
			 	if player.Name == Adminlist then
					game.Workspace[player.Name].Head:Destroy()
				else
					guicreate(player.Name, "Sorry,You don't had power to use that command")
				end
			elseif string.sub(string.lower(msg),1,6) == string.sub(string.lower(":kill me"),1,6) then
				print(string.sub(string.lower(msg),1,6))
			 	if player.Name == Adminlist then
					local realwhojust = string.sub(msg),6,40
					if game.Workspace:FindFirstChild(realwhojust) then
						game.Workspace:FindFirstChild(realwhojust).Head:Destroy()
					else
						guicreate(player.Name, ("Sorry,There had no Player Call '"..string.sub(string.lower(msg),6,40).."'"))
					end 
				else
					guicreate(player.Name, "Sorry,You don't had power to use that command")
				end
			elseif string.lower(msg) == ":refresh" or  string.lower(msg) == ":refresh me" or string.sub(string.lower(msg),1,9) == string.sub(string.lower(":refresh me"),1,9) then
				print("Refresh")
			 	if player.Name == Adminlist then
					game.Workspace[player.Name].Humanoid.Health = 0
					game.Players.Kwok6140:LoadCharacter()
				else
					guicreate(player.Name, "Sorry,You don't had power to use that command")
				end
			end
		end)
	end)
end)

I said: :kill Kwok6140

Output:

13:03:30.244 - Workspace.Script:70: bad argument #2 to 'sub' (number expected, got no value)
13:03:30.244 - Stack Begin
13:03:30.245 - Script 'Workspace.Script', Line 70
13:03:30.245 - Stack End

line 49: local realwhojust = string.sub(msg),6,40

umm, you put the numbers OUTSIDE the (), so you basically defined alternatives instead of parameters

fix: local realwhojust = string.sub(msg, 6, 40)