Question

    public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args){

        Player p = (Player)sender;
        Player z = this.getServer().getPlayer(args[0]);

        if(cmd.getName().equalsIgnoreCase("warn")){
            if(sender instanceof Player){
                if(p.hasPermission("warn.warn")){
//-----------------------------------------------------------------------------------------
                    if(!(args.length >= 0)){

                        p.sendMessage("§cFalsche Nutzung! &a/warn <Player> <Grund>");

                        return true;

                    }
//-----------------------------------------------------------------------------------------
                    else if(args.length == 0){

                        if(z == null){

                            p.sendMessage(Main.Prefix + "§cFehler! §aDer Spieler §2" + args[0] + " §aist nicht Online!");
                            return true;
                        } else {

                            p.sendMessage(Main.Prefix + "§cFalsche Nutzung! §aWieso willst du §2" + args[0] + " §awarnen?");

                        }

                    }
//-----------------------------------------------------------------------------------------
                    else if(args.length > 0){

                        String message= "";
                        for(int i = 1; i < args.length; i++){
                            message = message + args[i] + " ";


                        }
//                           MESSAGE
                        z.sendMessage("§8>----------------{ &6Warn §8}----------------<§r\n" + "§cDu hast einen Warn bekommen\n" + "§eGrund: §a" + message + "\n§8>----------------{ &6Warn §8}----------------<§r ");

                        return true;

                    }

                } else {

                    sender.sendMessage(Prefix + "§cZugriff Verweigert!");

                }
            }
        }
        return false;
    }