Please update this guide

[Matin-A]

Starting Discord.js 12 client.users, client.guilds, client.channels, guild.channels, guild.members are now a Manager instead of a Collection, so you can't use Collection.get(id) for them.

However you could use client.users.cache, client.guilds.cache, client.channels.cache, guild.channels.cache, guild.members.cache instead.

Please update this guide: https://github.com/AnIdiotsGuide/discordjs-bot-guide/blob/master/understanding/collections.md
Edit: See https://discordjs.guide/additional-info/changes-in-v12.html

[YorkAARGH]

If you look in the top left of the guide's website, there is a drop down selector where you can select version 12.