Originally Posted by
PTahCha
For economy seats, due to the move to dynamic pricing, unless the flight is sold out, every seat has a price, hence why you can't waitlist.
There's no difference in pricing strategies between Y awards and J awards. However, UA, per policy, does still allow waitlisting for J awards, and in this case they do specifically mean that you're waitlisting for I/IN classes, even if JN is available. The equivalent for economy would be a a waitlist for X/XN even if YN is available.
FWIW, only Premier members, or United / Chase credit card holders, have access to last-seat economy award inventory (YN). General members without a qualifying credit card can only access HN inventory.