Thank you for the question as I learned something new tonight. According to @Airbnbhelp, you have to "have completed a booking before you are able to have the Instant Book feature available."
Now, I am not too confident in this answer as I am pretty sure my very first booking was via Instant Book.
"thank you for contacting our Airbnb experts community. I am sorry if it was not clearly stated on the FAQ, but Instant book requires a profile photo and the completion of a stay with a positive review before it will appear on your menu. At that time you'll have the access to enable this feature. "
I thought that it is a guest need to have a positive review to book in instant book, not the host need a positive review to activate the instant book.