Turkish Airlines Elite Seat Selection Failed On Luanda Flight

I am a Turkish Airlines Miles&Smiles Elite member and on my flight on May 6 on the Luanda–Istanbul route I was unable to choose my seat during online check-in despite my Elite status. The system did not allow me to select a seat, and I could not benefit from the seat selection privilege that should come with my status. In this situation I could not find anyone to contact, so I was not able to receive any support or explanation from Turkish Airlines or Miles&Smiles regarding why I was prevented from choosing a seat. As an Elite member I expect an explanation and an apology for this experience, and as a gesture of goodwill I request an upgrade on my next flight on the Addis Ababa–Istanbul route.