Error Sending Message: Unsupported Post Request for Object ID '528471650346337'

Hello,
I am encountering an error when trying to send a message using the WhatsApp Business API. The error message states:
Sending of message failed. Unsupported post request. Object with ID '528471650346337' does not exist, cannot be loaded due to missing permissions, or does not support this operation. Please read the Graph API documentation at https://developers.facebook.com/docs/graph-api Here are the steps I followed to set up the phone number:
1- I added the phone number through the WhatsApp API setup. Verified the number by entering the code I received.
2-Added the phone number to my WhatsApp Business Account in the Meta Business Manager. The phone number status is now marked as "Pending."
3-I registered the number using the /register endpoint: POST https://graph.facebook.com/v21.0/528471650346337/register
The registration was successful without any errors. When I tried to send a message using the /messages endpoint, I received the above error. I am unsure what is causing this issue. Could it be:
Missing permissions for the access token I’m using? Misconfiguration of the phone number in my WhatsApp Business Account? An issue with the object ID or my API call setup? Here’s the API request I used to send the message:
curl -X POST "https://graph.facebook.com/v21.0/528471650346337/messages" \ -H "Authorization: Bearer " \ -H "Content-Type: application/json" \ -d '{ "messaging_product": "whatsapp", "to": "", "type": "text", "text": { "body": "Hello, this is a test message!" } }' Could someone help me troubleshoot this issue? Do I need additional permissions or configurations for the phone number to work correctly?
Thank you in advance!