I just remembered, sometimes you run into this problem when you try to book on an overbooked flight.

Let's say I or C show 0 and you're to get the discounted business fare. On Emirates website the seats look fully bookable, but when you go through the process you will get that error message either when you try and get the fare pricing (if you did search by schedule), or at the final confirm booking page (if you've done a price search).

Try changing your flights - if you're flexible. Or Book for a later date/flight, then call up Emirates and change the dates (if your fare conditions allow for this).
I suggest you also contact Emirates first, make sure there is availability in the booking class you're after.