In short#
I cannot see whether Meta can bill this account. After WhatsApp connected, open WhatsApp Manager and assign a payment method to this WhatsApp Business Account. A card on the Business portfolio alone is not enough.
Assign the payment method#
Open Settings → Messaging → WhatsApp → General.
When WhatsApp is linked, read the notice Check the payment method in WhatsApp Manager.
Choose Open Meta billing. Meta's billing hub opens.
In Meta, add a payment method if you do not have one yet.
Assign that method to this WhatsApp Business Account.
Meta screens change often. I describe the expected result, not each button. Direct link: Meta billing hub.
For linking the workspace, see Connect WhatsApp.
When I show a billing problem#
Paid messages and templates fail until Meta can bill this account.
I only show WhatsApp billing issue after Meta refuses a send (code 131042). If no send has failed yet, a missing banner does not mean the payment method is correct.
If the banner appears:
Choose Open Meta billing.
Assign a valid payment method to this WhatsApp Business Account.
Try the send again.
To send a template once billing is in order, see Send a WhatsApp template.