Who's operating the flight?
If it's an AC codeshare operated by UA from Canada to USA, you can't use UA web check in. Looks like it's only available for US departures only, from UA's site:
When can I use EasyCheck-in Online?
For U.S. domestic departures, including Alaska; Hawaii; San Juan, Puerto Rico; St. Thomas and the U.S. Virgin Islands, EasyCheck-in Online is available 1 to 24 hours prior to flight departure. For departures from the United States to international destinations, it is available from 2 to 24 hours prior to departure.
If it's an UA codeshare operated by AC, you need to get the AC locator before you can web check in, the UA locator will not work with AC web check in.