Sounds strange to me. Just looked up some random flight EWR-DEN on United and for sure there is availability in X ‘around’ December 2019/2020. Not that much though... busy time!

both on the United Website and on the SAS Star Alliance Award booking tool.

it might be, there was no flight in the date (or like +/-2 days) and the agent thought they were not loaded.

best option ist to go on the Unites website, search for availability and then verify what you found in the SAS Star Alliance Award booking tool. The call in.