Hi there Hend

Online check in can fail to work for a wide variety of reasons. But just remember that you don't *need* to check in online, it's just an option. And if you have preselected seats and you can see your ticket online in Manage my booking there is actually little benefit to you as the passenger in doing so. If you absolutely need some confirmation you probably need to call QR to confirm what is going on. Otherwise you really do not need to unduly worry and just turn up at ICN about 3 hours before your scheduled departure time and just check in at the counter as usual and enjoy the rest of your time in Korea.