It looks like possibly avios bookings on S7 have been disabled from ba.com. It certainly was possible. I last booked IKT-DME for avios in June, so it was working.

Also, if you look for DME-MAD it only shows the IB flights, not the S7 ones. So I think just something is broken at the moment. There is normally plenty of availability on S7. Standard 2 seats in J on any route as I recall.