Ehy,
the issue happens because you’re using the test webhook. In test mode, n8n creates a temporary webhook that only works once or for a short time and then returns 404.
To fix it:
Activate your workflow by switching the toggle to ON in the editor.
Use the production webhook URL (without -test) and update it in your Facebook Messenger app settings.
Re-run the subscription/verification step on Facebook if needed.
Let me know if this solves your problem and mark the response as a solution.
In this way, you encourage users to give more and more punctual and precise answers within the community