Why can't I use a number which is already using WhatsApp? I read that WhatsApp does allow that.

Yes, WhatsApp will allow adding existing WhatsApp Business/Consumer App (Android/IOS) phone number to a WABA, but won’t allow downloading a certificate for docker setup until the phone number and an associated account is deleted from the Business/Consumer app. Therefore we have not provided this provision on Gupshup's WhatsApp tools and APIs