It sounds like the UA agent made a mistake. (Edit to add: but, having read the posts below, I am now pretty sure that the mistakes were made by the people who told OP that this would be okay.)

Just out of curiosity, were you also planning to fly back from Canada to the U.S.? Did you think that would be OK with just a birth certificate?

(And adding: what do you want to happen here? Do you want a refund for your trip? Additional compensation for nonrefundable trip cancellation costs? UA to put you on the next available flight, possibly on another airline? It sounds like the best thing to do here is to get someone on the phone who is on your side, i.e. the same UA 1K reservations agent who confirmed that a birth certificate was sufficient for UA for travel to Canada, and have them talk directly to the person who is not putting you on the plane.)