Discord error 50001: Missing Access

What causes it

The bot cannot view the channel. Without View Channel — granted server-wide or in that channel's own permissions — the channel does not exist as far as the bot is concerned, and sending, reading or fetching anything in it fails with 50001. Private channels and categories are the usual culprits. For voice channels, Connect is needed as well.

The bot is not in the server. Fetching a server, channel or member by ID after the bot has been removed from that server gives Missing Access, as does a hard-coded channel ID copied from a different server.

The invite left out slash commands. Registering commands in a specific server fails with 50001 if the bot was added without the applications.commands scope. Re-invite it with a link that includes both scopes; there is no need to remove it first.

It is a private thread the bot has not joined. The bot needs to be added to the thread, or to have Manage Threads, before it can post in it.

Finding which one

Log the channel or server ID alongside the error, then look at that exact channel's permissions with Discord's "View server as role" feature on the bot's role. If the channel vanishes from the list, it is a visibility problem.

const { PermissionFlagsBits } = require('discord.js');

const channel = await client.channels.fetch(channelId).catch(() => null);
const canSee = channel?.permissionsFor(client.user)?.has(PermissionFlagsBits.ViewChannel);

if (!canSee) console.warn(`Cannot see channel ${channelId}`);