How are you "purchasing" the ticket? If you're using a flight pass and on some non-consumer channels, an Aeroplan account is necessary. In which case you'll create an Aeroplan account, book the flight then go into manage reservations to update your frequent flyer back to UA. I do this all the time for my flight pass bookings and haven't experienced any issues accessing my UA Gold benefits nor getting milage accrual on the flights.

If you're using the normal AC website, as others have noted, there should be a continue as guest option.

Hope that provides some insight.

Safe Travels,

James