Tried to use the website today to book a Premium Economy ticket paying with real money DEN-LHR in April. I got to the payment page and when I tried to pay, repeatedly got the error "We could not process your request. Please try again later or call customer service....". I called customer service and the rep told me I was the second person she had had call with an issue buying premium economy from the website. The rep wondered aloud if, although the website gave a price for premium economy and permitted me to select seats for premium economy, if part of the website thought that my flight (and other flights) didn't have PE seats so wouldn't process the purchase. The rep could purchase the PE ticket for me and didn't charge me the $20 fee for booking by phone because of the website problem. FWIW, I use brave as my browser but I suspect that's not related.

Mentioning it only because, since at least one other person seems to have the same issue, others reading here might too.

I agree with everyone else: one would have thought UA would rush to fix anything that stops people from paying them money but apparently, one would be wrong.