An internal error occured while attempting to perform this command

Hi, i'm trying to make my own command to set someones/your level in-game. But when i use the command in-game it says "An internal error occured while attempting to perform this command". Currently i'm aiming at changing the game level but i'd like to make a sort of a rpg plugin later on. I just started with java and before posting this i searched the forum & tried to fix it myself for an hour.

Code:

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
      if (command.getName().equalsIgnoreCase("levelset") && sender instanceof Player) {
      
        Player player = (Player) sender;
        int intendedlevel = Integer.parseInt(args[0]);
        if (args[1].length() == 0) {
            player.setLevel(intendedlevel); }
        else {
        Player target = Bukkit.getPlayerExact(args[1]);
        String targetname = target.getDisplayName();
        targetname.setLevel(intendedlevel);
        player.sendMessage(ChatColor.RED + "Level of player" + args[1] + "set to" + intendedlevel);
        return true;
        }
      } else {
        sender.sendMessage(ChatColor.RED + "You are not permitted to do this.");
      }
    sender.sendMessage(ChatColor.RED + "This command does not exist.");
    return false;
    }

Thank you for your time and sorry for asking you to help me with what is probably a very trivial problem.
Also i have one more question, into

Code:

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

can be put more than one commands, right?

 

Go into the server console and post the error message.