Test/unverified number: while the Meta app is in Development mode, the phone number can only message recipients added to the allowed recipient list (Meta → WhatsApp → API Setup → "To").
24-hour window: a free-text message is only delivered if the customer wrote to you in the last 24h. Otherwise you must use an approved template.
Wrong format: numbers must be international without "+" or leading 0 (e.g. 8801785466804, not 01785466804). The form fixes this automatically.
Payment method / business verification missing on the WABA.
The real reason is reported by Meta in the delivery status shown above and in the Logs tab.
Event log
Every webhook call from Meta (incoming messages, delivery statuses), every send attempt and every error — newest first.