Originally Posted by
Kacee
I've already seen similar restrictions. For instance, HNL-PHX on Dec. 5, both HNL-SFO and SFO-PHX have Z inventory, but the combined itinerary is Z0, because UA only wants to sell a D or higher for that routing. And there's actually no way to buy that Z on a single ticket - both google flights and the website (using multi-city) will price it (more than $300 less than the D), but UA will not issue the ticket.
The description strikes me as matching the issue raised here:
Ghost Inventory?
Get all the way through to purchase, but system refuses to ticket.
(edit add: given your example, I'd have to revise my opinion in post 14 of that thread; seems intentional, then.)