Originally Posted by
horse glasses
if I do say LAX-YYZ on UA and then YYZ-YYT on AC, the error will pop up when I pick the connecting flight.
Are you trying to book a stopover in Toronto? If so, UA would not be allowed to sell the YYZ-YYT segment (even if operated by AC) due to cabotage laws. You would have better luck booking on aircanada.com.