Turkish Airlines Did Not Issue a Boarding Pass for a Valid Ticket

I bought a Turkish Airlines ticket for my brother from Ucuzabilet.com. I checked in. When I wanted to create a boarding pass, I saw a message like "this boarding pass cannot be issued online, it must be obtained at the airport". When my brother went to the airport, they told him "we cannot see our check-in screens, so we cannot issue your boarding pass". It is impossible for him to board the plane without a boarding pass. That is why we had to buy a new ticket. I wrote to Ucuzabilet, they gave an irrelevant answer, it seems like they are trying to say "we only sell the ticket, we do not get involved in the rest". We were seriously victimized for a reason that was not caused by us.