They keep moving it around and renaming it, plus now it's only available if you're logged in.
I just tried a test booking
while logged in, and it's now earlier, on the Rental Options page, which includes Equipment/Services, then Protections/Coverages, then Frequent Travel Program For Miles/Points. That last section is what you're looking for. You have to type in what you think the name of the program is and it'll try to match it as you type; you have to spell it out (for example, American, not AA).
It used to be later in the reservation process.
I just repeated the same trial booking while
not logged in. Got a slightly lower price that way, but the section for Frequent Travel Program is missing. The disappearing act for the FFP field was first brought up a few months ago as
a Budget issue, and that's when I discovered that it works at Avis
only if you're logged in. Fortunately, joining Avis Preferred and thus getting a login is free.