This is a common situation with many airlines. Ticket is there, you’ve paid for the flight and all. But, there is no PNR, so there is no record of you on the flight and there is no seat reservation. The PNR likely got nuked as a valid ticket was not associated with it before it auto-cancelled (initially or when rebooked/changed). In such a scenario impossible to check in. If flight is not full you may be able to get on in whatever seat is left over as standby. Better to get it fixed timely. Here DL has to create the pnr and then associate the existing ticket (if valid) with it, that’s all. Being that DL is DL they will, of course, refuse to that and send you to the ticketing carrier, and while that is rightly their problem to solve, that is much harder as they need to redo all from the start, or otherwise still get DL res to help manually. Quickest would be for DL to create pnr and associate the other carrier ticket with it, if valid. That’s usually the best way to solve these problems.