I checked using 'business' in the search option and that flight didn't come up. Could it be that they've upgraded some Gold/Silver/etc passengers from economy? When I looked at the seat plan for economy, the flight was practically empty.
It's very unlikely that this has been caused by anyone being upgraded for free. And seat maps are generally useless for trying to guess loads and booking levels. Most of the time, you'd do just as well to read tea leaves.
If you can book in economy but not in Club, then at the moment all Club inventory is booked or is off sale for some other reason. This may or may not change but you do have the option of asking all the way up to check-in at the airport.