（节点：656）UnhandledPromiseRejectionWarning：ReferenceError：未定义前缀错误

Question

我目前正在尝试让Discord机器人在Discord中播放音乐，我已经键入了所有这些代码，并且出现了以下错误：

（节点：656）UnhandledPromiseRejectionWarning：ReferenceError：前缀未定义

我已经尝试修复它，但是不能。我正在使用JavaScript，而我正在使用的程序是Visual Studio Code。

这是代码：

    const {Client, MessageEmbed} = require('discord.js');
const bot = new Client();

const queue = new Map();

const ytdl = require("ytdl-core");

const token = 'HIDDEN FOR PRIVACY';

const PREFIX = '!';

var version = "1.0.0"

bot.on('ready', () =>{
    console.log('Krum has started');
    bot.user.setActivity(`!help | Krums Bot`);
})




//ATTEMPT AT PLAYING MUSIC--LOOKING FOR FIX--NOT WORKING CURRENTLY
bot.on("message", async message => {
    if (message.author.bot) return;
    if (!message.content.startsWith(PREFIX)) return;

    const serverQueue = queue.get(message.guild.id);

    if (message.content.startsWith(`${PREFIX}play`)) {
      execute(message, serverQueue);
      return;
    } else if (message.content.startsWith(`${PREFIX}skip`)) {
      skip(message, serverQueue);
      return;
    } else if (message.content.startsWith(`${PREFIX}stop`)) {
      stop(message, serverQueue);
      return;
    } else {
      message.channel.send("You need to enter a valid command!");
    }
  });

  async function execute(message, serverQueue) {
    const args = message.content.split(" ");

    const voiceChannel = message.member.voice.channel;
    if (!voiceChannel)
      return message.channel.send(
        "You need to be in a voice channel to play music!"
      );
    const permissions = voiceChannel.permissionsFor(message.client.user);
    if (!permissions.has("CONNECT") || !permissions.has("SPEAK")) {
      return message.channel.send(
        "I need the permissions to join and speak in your voice channel!"
      );
    }

    const songInfo = await ytdl.getInfo(args[1]);
    const song = {
      title: songInfo.title,
      url: songInfo.video_url
    };

    if (!serverQueue) {
      const queueContruct = {
        textChannel: message.channel,
        voiceChannel: voiceChannel,
        connection: null,
        songs: [],
        volume: 5,
        playing: true
      };

      queue.set(message.guild.id, queueContruct);

      queueContruct.songs.push(song);

      try {
        var connection = await voiceChannel.join();
        queueContruct.connection = connection;
        play(message.guild, queueContruct.songs[0]);
      } catch (err) {
        console.log(err);
        queue.delete(message.guild.id);
        return message.channel.send(err);
      }
    } else {
      serverQueue.songs.push(song);
      return message.channel.send(`${song.title} has been added to the queue!`);
    }
  }

  function skip(message, serverQueue) {
    if (!message.member.voice.channel)
      return message.channel.send(
        "You have to be in a voice channel to stop the music!"
      );
    if (!serverQueue)
      return message.channel.send("There is no song that I could skip!");
    serverQueue.connection.dispatcher.end();
  }

  function stop(message, serverQueue) {
    if (!message.member.voice.channel)
      return message.channel.send(
        "You have to be in a voice channel to stop the music!"
      );
    serverQueue.songs = [];
    serverQueue.connection.dispatcher.end();
  }

  function play(guild, song) {
    const serverQueue = queue.get(guild.id);
    if (!song) {
      serverQueue.voiceChannel.leave();
      queue.delete(guild.id);
      return;
    }

    const dispatcher = serverQueue.connection
      .play(ytdl(song.url))
      .on("finish", () => {
        serverQueue.songs.shift();
        play(guild, serverQueue.songs[0]);
      })
      .on("error", error => console.error(error));
    dispatcher.setVolumeLogarithmic(serverQueue.volume / 5);
    serverQueue.textChannel.send(`Start playing: **${song.title}**`);
  }