I am receiving the following error message - "Phone Number is already in use. Try using a phone number that isn't connected to a WhatsApp account." What should I do?

You will receive this error when the phone number you are trying to register is currently active/registered for WhatsApp's business/consumer app. WhatsApp does not allow a phone number to be registered for multiple WhatsApp platforms i.e. you cannot register a phone number for WhatsApp consumer/business app and WhatsApp Business API simultaneously.

To register the number for WhatsApp business API, you will have to migrate the number from WhatsApp business/consumer app to WhatsApp business API.