[Niko_boy]

pawn Code:

public OnPlayerCommandText(playerid,cmdtext[])
{
 //---- all commands
  return 0;
}

down there it is return 0 make it

pawn Code:

return SendClientMessage(playerid,-1,"Invalid COmmand");

So it returns That message where " -1 " is color you want , and Replace Invalid Command by message you want.