Your seat assignment could be canceled if you dont check in within the time limit. Check-in times are based on security requirements and vary depending on where youre. In these cases, the other airline will likely be unable to assign a seat for you in advance if you ticketed your reservation with United.
https://www.united.com/ual/en/us/fly/help/faq/aircraft-seating.html
...Airlines Shenzhen Airlines Singapore Airlines South African Airways SWISS TAP Portugal THAI Turkish Airlines United.. Advanced seat assignment. During your flight, would you like to sit by your friends or family. Advanced seat assignment is available at no charge for the following passengers
https://www.copaair.com/en/web/gs/advanced-seat-assignment
United has desperately been trying to generate more incremental revenue. Here's their latest move, which gives basic economy passengers more options.. So, how much is United charging for seat assignments on basic economy fares? It will vary by flight, but for a Tampa to Chicago flight for later...
https://onemileatatime.com/united-basic-economy-seat-assignments/
First up the skinny on United Airlines seat selection! Basic Economy Basic Economy is Uniteds most inexpensive fare think of it as your bare bones. Economy An Economy fare on a United flight allows you to self-select your seat and travel with a personal item plus carry-on at no extra charge.
https://www.cheapair.com/blog/?p=800503591
You should be able to call Air Canada to get seat assignments...And perhaps do through their website if you have the AC confirmation number--should be listed at the top of your itinerary when viewing it on united.com.
https://www.flyertalk.com/forum/united-airlines-mileageplus/1625105-how-do-i-get-seat-assignment-air-canada-flight-booked-united.html
United Airlines has used the power of social media to express a good point well made. The Points Guy reports that a customer by the name of Krunal Patel was aboard a United Airlines domestic flight when he contacted them via Twitter in order to complain about his seating arrangements.
https://simpleflying.com/united-car-economy-plus/