AE seems to have some weird issues with CA ticketing.
A couple of months ago, I tried to book an itinerary that included CA PEK-SIN. It showed up okay online. Selected the flight. Wouldn't let me book.
Replace CA PEK-SIN with SQ PEK-SIN, and suddenly the button to book goes active again.
And then about a month ago, booked online (and then flew) an itinerary that includes CA SIN-PEK, no problems.
They never cease to find new says to boggle the mind.