Spraw, aby bot wysłał wiadomość na kanał tekstowy, gdy ktoś dołączy lub opuści jego kanał głosowy

Myślę, że to nie działa, ponieważ używasz bot.voiceChannel, ale to nie istnieje: .voiceChannel jest własnością GuildMember, więc najpierw musisz zdobyć członka.
Zrobiłbym to tak:

bot.on('voiceStateUpdate', (oldMember, newMember) => {
  // Here I'm storing the IDs of their voice channels, if available
  let oldChannel = oldMember.voiceChannel ? oldMember.voiceChannel.id : null;
  let newChannel = newMember.voiceChannel ? newMember.voiceChannel.id : null;
  if (oldChannel == newChannel) return; // If there has been no change, exit

  // Here I'm getting the bot's channel (bot.voiceChannel does not exist)
  let botMember = oldMember.guild.member(bot.user),
    botChannel = botMember ? botMember.voiceChannel.id : null;

  // Here I'm getting the channel, just replace VVV this VVV with the channel's ID
  let textChannel = oldMember.guild.channels.get('CHANNEL_ID_HERE');
  if (!textChannel) throw new Error("That channel does not exist.");

  // Here I don't need to check if they're the same, since it would've exit before
  if (newChannel == botChannel) {
    // console.log("A user joined.");
    textChannel.send(`${newMember} has joined the voice channel.`);
  } else if (oldChannel == botChannel) {
    // console.log("A user left.");
    textChannel.send(`${newMember} has left the voice channel.`);
  }
});