Dueling System, completely broken. Any clues on how to fix this?

Haven't played ROBLOX since 2014 really and a lot has changed, so could someone help me with what is going wrong here?

function onChatted(msg,speaker) 
if (string.sub (msg,1,10) == "duel/" then ---you say this first and then the players name
p = game.Players:GetChildren() ---finds player
for i=1, #p do ---the players that to start
if p[i].Name == string.sub(msg,11) then ---the player name
m = Instance.new ("Message") ---creates a message
m.Parent = p[i].Character ---sends it to the requested player
m.Text = ""..speaker.Name.."has challenged you! Type accept/ to Fight or Decline/ to decline " ---speaker.Name is the player requested
wait(30) ---waits 30 seconds for it to reply, if he/she doesn't it removes
m:Remove() 
end 
end ---ends the script/line.
end 
if (string.sub (msg,1,7) == "accept/") then ---if the player accepts...
p = game.Players:GetChildren() 
for i=1, #p do 
if p[i].Name == string.sub(msg,8) then 
if (m ~= nil) then 
m:Remove() 
speaker.Torso.CFrame = CFrame.new (Vector3.new (109.521, 76.298, -246.288)) ---change these numbers on where the speaker teleports
f = Instance.new ("Message") 
f.Parent = p[i].Character 
f.Text = ""..speaker.Name.."has accepted your challenge!" ---message sent to the speaker :D
wait(2) 
f:Remove() 
p[i].Character.Torso.CFrame = CFrame.new (Vector3.new (114.04, 75.519, -272.141)) ---change these numbers on where the other player teleports
end 
end ---ends?
end 
end 
if (string.sub (msg,1,8) == "decline/") then ---decline? Hes scared xD
p = game.Players:GetChildren() 
for i=1, #p do 
if p[i].Name == string.sub(msg,9) then 
z = Instance.new ("Message") 
z.Parent = p[i].Character 
z.Text = ""..speaker.Name.."has declined your challenge!" ---sends to the requester
end 
end 
end ---ends the whole entire script
end 

function onPlayerEntered(newPlayer) ---makes this work with every player that enters
newPlayer.Chatted:connect(function(msg) onChatted(msg, newPlayer) end) ---connects with the players
end 

game.Players.ChildAdded:connect(onPlayerEntered)