The Twilio Account contains other WhatsApp Senders and is already linked to an existing WhatsApp Business Account (WABA). Twilio Accounts must be linked to only one WABA ID at a time.
Possible Causes
Attempting to use a different WABA ID than the one linked to the Twilio Account
Possible Solutions
Use the same WABA ID that is linked to the account
Delete other WhatsApp Senders to link the Account to another WABA