Just seeing this…. Same issue as OP. FWIW I tried loads of searches yesterday and couldn’t get the usual upgrade info to come up either. I chalked it up to weekend United.com issues. Seeing this post prompted me to do another search and same thing. The little buttons that indicate availability or waitlist don’t even show up. Yet I see other posters for whom it appears to be working.

Tried two browsers on two different computers.