In order to use the WhatsApp Business API with an existing WhatsApp number (account) you first have to delete the WhatsApp account in the mobile app.
You can delete your account from within WhatsApp. To delete the account:
- Open WhatsApp.
- Tap More options > Settings > Account
- Select Delete my account.
- Enter your phone number in full international format and tap DELETE MY ACCOUNT.
Deleting your account will:
- Delete your account from WhatsApp.
- Erase your message history.
- Delete you from all of your WhatsApp groups.