I read recently on another thread in this forum that it's not possible to activate Instant Booking until one has already had a completed booking.
This might be the case here.
What I read was:
"According to @Airbnbhelp, you have to 'have completed a booking before you are able to have the Instant Book feature available'."
I'd advise you, though, to be use IB prudently. Perhaps familiarise yourself first with the various options (e.g. having IB available only a possibility for guests who have been verified by AirBnB and recommended by other hosts).
If you post a query in the 'search the community' box, a number of threads will be sure to appear.
