I am unable to add phone number on the UI it says - "Phone Number is already in use. Try using a phone number that isn't connected to a WhatsApp account." What should i do?

You can receive such a message when the phone number you are trying to register is already active on WhatsApp business/consumer Andriod/iOS app.

Make sure you comply with following:

1. The phone number should be never used for WhatsApp Business API and should not be active on WhatsApp Consumer/Business app(Andriod/iOS) while registering with via Gupshup
2. If the phone number was used on the WhatsApp app (Andriod/iOS), delete the account and try again (usually it take 7 days to clear cache and then you can retry)