It’s probably an availability issue. You will get an error if the website is unable to find available space on all the legs of the flight for that particular date. So the best way to search is segment wise. Make sure your PDX-SFO, SFO-CCU, CCU-SFO, SFO-PDX are all available for your travel dates.

You can use the UA site to search for saver availability as well esp. for the UA flights.

If the PDX-SFO leg is not available on UA you have a few choices - 1. Pay for the PDX-SFO leg separately. 2. Call SQ and ask them to check if there is availability on AS on the PDX-SFO leg - that’s not searchable through the website - which will mean more miles to pay.