Modified your code

public class LobbySystem extends JavaPlugin implements Listener {
    private void registerCommands() {
        getCommand("nick").setExecutor(new NickCommand(this));
    }
}


public class NickCommand implements CommandExecutor {
    private LobbySystem pl;
    public NickCommand(LobbySystem main) {
        this.pl = main;
    }
    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String cmdlable, String[] args) {
        if(sender instanceof Player) {
            Player player = (Player) sender;
            if(args.length == 0) {
                pl.api.setRandomPlayerNickName(player, "", "");
                pl.api.setPlayerChatName(player, pl.api.getNickName(player), "", "");
                pl.api.setPlayerDisplayName(player, pl.api.getNickName(player), "", "");
                pl.api.setPlayerTablistName(player, pl.api.getNickName(player), "", "");
                pl.api.setRandomPlayerSkin(player);
            } else if(args.length == 1) {
                String nickname = args[0];
                pl.api.setPlayerNickName(player, nickname, "", "");
                pl.api.setPlayerChatName(player, nickname, "", "");
                pl.api.setPlayerDisplayName(player, nickname, "", "");
                pl.api.setPlayerTablistName(player, nickname, "", "");
                pl.api.setRandomPlayerSkin(player);
            }
        } else {
            sender.sendMessage("Fehler!");
        }
        return true;
    }
}