For the benefit of others, I thought it'd help to restate and consolidate some rules people have identified (Richklhs and others) about ANA award booking via Etihad Guest.
* Availability can be checked via United's Mileage Plus award booking engine
* Must be round trip booking, no one-way bookings
* No open-jaw (must arrive at, and depart from, the same airports)
* Booking required to be 14 days prior to travel (may be flex here)
* Must hold points required in order to finalise booking
* Taxes and Surchages paid in currency of first city of departure
* One free change allowed
* Points forfeited if no show