This message comes up if the selected flight has no availability online, even if other flights that day are open.

I was successful booking such a flight on the phone with FB (after 6 calls, ~30 minutes). On the other hand, another flight which was available online was deemed to require Opti-miles on the phone.

My France-based agent, while helpful and thorough, did not offer any KL flights until asked. Surprisingly, taxes on this Asia-GVA routing were about half as high through AMS compared with CDG.