SunExpress Online Check-In Error Blocks Me From Sitting With My 5-Year-Old Child

For my September 11, 05:30 Antalya–Düsseldorf flight with SunExpress (reservation number XE22EB), I have repeatedly attempted to complete online check-in but at the final step I continuously receive the error “psoperation Exception.” Since my flight is tomorrow and I am traveling with my 5-year-old daughter, it is urgent that I check in online so we can sit together. I tried multiple times using both computer and phone, with Chrome and Safari browsers, yet the same error persists. I also contacted SunExpress customer service, but they only told me that if check-in is done over the phone, I would be charged an extra fee, and assured me that there was no problem with cookies or the system. Despite this, all of my attempts have failed, leaving me unable to secure seats with my child. My ticket is fully paid, and I did not agree to any extra services during booking. I urgently request that SunExpress resolve this issue immediately so I can complete my online check-in without errors and ensure that I am seated next to my daughter on this flight.