If you want the OLCI bonus, print out the page saying that you cannot, and call Mileage Plus so that they can put a note in your account so that you can send them that page (basically, you attempted in good faith to OLCI).

Once it happened to me. The UA person said that it was because someone else with the same name was on the flight. (Fortunately, the flight originated at a UA outpost, so the checkin line was only a few minutes, instead of the 50 minutes or so at a UA hub.)