I spent quite a bit of time last night attempting to book DEN-KTM (Kathmandu, Nepal) on United's website.
Any itinerary with KTM as the destination immediately returns "We couldn't find a flight but we can still help" error screen.
As experiments, I tried more direct routings like EWR-KTM, EWR-DEL-KTM, ORD-DEL-KTM, DEL-KTM, but all instantly come back with the same error page. All (US)-DEL searches came back as expected.

This is somewhat surprising to me since Air India partner serves DEL-KTM, and pseudo-partner Vistara also does (they have a UA codeshare agreement, but admittedly I believe the KTM flights are not in that group).

Called the premier desk and the agent was able to manually create the itinerary (using the Vistara flight numbers for DEL-KTM) I fed over the phone but said it would have to be sent to the rate desk. She said read some notes in the system about Air India being blocked because their system was a mess, but it sounded like she was talking about Award tickets. Said I would think about it and call back later.

(as an aside, after some more messing around, at this point I tried doing the "text conversation an agent" thing, biggest waste of an hour of my time ever, big regret).

A different agent this morning was able to do it manually without using the rate desk but she assumed the error was on my part (that I should use multi-city search), but I wanted to post this out there for curiosity to know what might be going on here.

So, conclusion is tickets successfully booked on one itinerary (separate confirmation number for Vistara DEL-KTM, $25 fee waived), but I was curious if I'm missing something here.