Did you end up either figuring it out online or getting the call center to book this? The error you're getting in the booking flow is usually an availability failure. The actual error code seems to vary but from what I've seen, between pax info and seat selection, the web engine does a final availability check and fails if it can't confirm. The mismatch is usually because matrix usually checks availability on a segment basis (especially if you're specifying a multi city booking or very specific routing codes) and the engine reconfirms it using O/D availability and married segment availability. I think (but I'm not sure) that error can also be thrown with certain carriers that the web engine can't pull live availability from. If that is what was happening, calling in could get it done.