FAQ
Do I Need Business Verification & Approval From WhatsApp?
No it is not required.
why i can't reply / send messages to user as reply in admin chat area specially when i try after one day
Facebook allows replies through Cloud API within 24 hours of the received status timestamp.
Can i send whatsapp messages if my business is not verified on FB?
Limited to 50 free messages per day without Business verification of the Facebook App (this limitation doesn't exist if you verify your Business) and to generic WhatsApp Cloud API Usage Policy