I also got this error for my recent booking. Waited ~2 weeks and tried again and was able to select seats. There may be a lag in the system from when tickets are available on Air France to when you can select on Czech Airline's website.

Another note, I found I could only select window seats. My partner and I booked on separate reservations but cannot select the middle seats. It may be similar to Qatar where we had to call in to get those seats as they're reserved for people traveling together.