Discord error 50021: Cannot execute action on a system message

The message you targeted is a system message, one generated by Discord rather than sent by a user, bot or webhook: member join notices, boost announcements, pin notifications, thread-created markers, channel follow adds and similar (message types other than 0 DEFAULT and 19 REPLY, with some exceptions). Certain actions are not allowed on them, notably pinning and replying with a message reference in some cases.

Bots see this when they blindly act on every incoming MESSAGE_CREATE. A welcome bot that reacts to the "X joined the server" system message is fine (reactions are allowed), but one that tries to pin it or reference it in certain ways is not. Auto-pin and auto-thread features should check the message type first. Note that system messages can be deleted with Manage Messages; it is mainly pinning that is refused.

{
  "message": "Cannot execute action on a system message",
  "code": 50021
}

Make message type a first-class filter in your event pipeline: most bot features should only ever look at type 0 and 19. Document the exceptions (for example reacting to join messages) explicitly rather than handling all types by default.

System messages are the platform talking to the channel, and the API mostly treats them as read-only furniture. Filtering by message.type at the top of the event pipeline is cheaper than special-casing each feature; the message-type list lives with the message limits on the Discord limits page, and pinning real messages still requires the pairing rules in 50019.