non functional landing page issue

You should double-check that WhatsApp Business is working, has a complete profile, is not restricted, has the correct country phone number, and that the Page is properly linked to WhatsApp in Business Manager.

This can happen when Facebook’s automated review system fails to load the WhatsApp chat correctly, which may cause the ad to be flagged as having an invalid or unreachable destination, even if no website link is involved.

Can you share a screenshot of the ad setup?
The issue is likely in the link parameters. Even with direct WhatsApp, FB checks if the linked number is active and if the business profile is fully set up. You can try to verify your W Business account and ensure the link is formatted correctly (wa.me/...)