Sending of message failed. Account does not exist in Facebook Hosted API, please use /register API to create an account first.

You need to first register the number on WhatsApp Cloud API before you can begin messaging. Checkout their developer docs: https://developers.facebook.com/docs/whatsapp/cloud-api/reference/registration

Note, you will need to add the number to the Business Manager account and verify it in the process first.

Copy the Base64-encoded Certificate

A base64-encoded certificate string is generated for you and is used for validation during WhatsApp Business API client registration.

1. Go to the Business Settings in your Facebook Business Manager.
2. Click on WhatsApp Accounts under the Accounts menu, then select your WhatsApp Account from the WhatsApp Accounts list, if it isn’t already highlighted.
3. Go to the Settings tab, and click the WhatsApp Manager button.
4. Under the Phone Numbers tab, you should see your phone numbers listed. Click the View button of the phone number you want the certificate for.