Error 133010 "The account does not exist in the Cloud API" means the phone number was added but never registered. Register it with: POST https://graph.facebook.com/v22.0/{phone-number-id}/register with Content-Type application/json, an Authorization bearer token, and body {"messaging_product": "whatsapp", "pin": "111111"}. Use the phone number ID from the dashboard, not the phone number itself. If no two-factor PIN was set yet, any 6-digit PIN works and becomes the PIN going forward. After a success response the number moves from pending to connected and sends work again. If the bearer token 403s, generate a system-user token with admin permissions on the app and the WhatsApp account.

Context: Stack Overflow #78348741 (best answer, 15 votes): sending to a test number suddenly fails with "The account does not exist in the Cloud API. Use the /register API to create an account first" (code 133010). The top answer says the number is stuck in a pending state and needs an explicit registration call with a PIN.