WhatsApp API is valid only for registered businesses. Facebook will ask you to send the legal documents related to this. The review and verification can take up to 15 days, so it is recommended that you do this in time.
Facebook verification is generally approved for notable businesses and not for smaller ones. Make sure your business is and that it complies with Facebook's terms and conditions for merchants and businesses. After verifying your account with Facebook, you'll need to complete a few important steps in order to successfully migrate your account from WhatsApp Business to WhatsApp Business API.