In my experience when I see this message looking for a discounted business class fare internationally it means that the leg in question will be ticketed with a "Y" fare code and not C, Z, or J. If there's I space (unlikely) you can then upgrade with a cert at the time of booking, but if there isn't you'll be playing Aerolotto at the gate.

Check the actual class shown at the right side of the selection screen to be sure, but I've never seen a case where the AC website will sell you a leg with a J, C, or Z fare code and then only give you an economy seat.

This issue only seems to come up on routes where you get the "old" version of the selection screen with the two tabs for Executive class: Flexible, and Lowest Price.