This is a solution to the question originally posted by @msanta7 here. Similar issues where raised here and here (maybe not exactly the same but it is useful to keep them all mentioned in this topic)
Question was closed with useful information yet the actual problem came from Facebook Graph API not recognizing the application created in Meta developer platform.
This video explains perfectly the steps to follow, it is in Spanish (even without subtitles you can work out each step)
Essentially, once all credentials are set correctly in the n8n whatsapp trigger and in facebook (app, portfolio, permissions, etc), your new application needs to be “enabled” via the Graph API, otherwise the trigger will not work even if messages are received and seen in the developer platform.
From the developer.facebook.com panel, copy your WhatsApp Business Account Id(you get it from the Basic Configuration - Step 1, where you see the test phone number).
On the top right corner, the Tools tab opens a dropdown where you can access the Graph API Explorer.
After you’ve generated an Access Token, you must make a POST request to https://graph.facebook.com/v25.0/<your whatsapp business account id>/subscribed_apps
With "success": true response, your whatsapp trigger should now listen to messages without problem.
Hope this helps, it would be great that the n8n docs mention this in some manner, since Meta developer docs don’t mention it.