AC promo codes only work for AC metal. Try booking from PEK and you should have no problems. The short first leg will have to be booked on a separate ticket.
That's not exactly true. For example, this is bookable with a promo code.
The difference between this and the earlier CA flight is that this one has an AC flight number, and the earlier flights don't.
The terms say
The discount applies to the base published fare for Air Canada, Air Canada Rouge and Air Canada Express designated flights.
I guess non-AC metal is ok as long as there is an AC flight number.
I figured this part out, but I still don't understand why there is no business class available if booking CAN-PEK-YVR but it's available for CAN-PEK-YVR-YYC or CAN-PEK-YVR-(other Canadian cities).
If I look at the booking class codes for a booking without a promo code, CAN-PEK-YVR books into Z class, while CAN-PEK-YVR-(other Canadian city) books into P class.
So it appears that the promo code cannot be used for Z class, even though that is not mentioned anywhere.
Regarding booking from PEK directly, those flights are much more expensive. I think AC is offering good prices for business class from CAN to compete with CZ direct CAN-YVR flights.