1. It is necessary to make sure that the data entered on the site is correct.
2. Probably your airport does not have an online check-in option.
3. You are trying to check in an unaccompanied child or passenger on a stretcher or have paid for an extra seat.
If you were unable to check in online, please do so at the airport check-in desks. Some airports (currently Almaty and Astana airports) have a self-check-in kiosk you can use in the terminal.