Is it possible your SAS codeshare booked into DL’s basic economy (E)? If so, no advance seat selection is possible.

Also just a possibility of weird codeshare issues - it’s not uncommon for this to happen with two airline’s IT not talking to each other properly.

I was considering a fall trip to BKK as I came across a nice DL PS fare, but the resulting KE economy segment wanted to book in their version of basic, with no advance selection possible. Decided to pass for multiple reasons, but that was a factor. Normally if I buy J seat selection is no issue, but the J fares were insane.

I’d probably call back and see if another agent has more luck, and at a minimum confirm that you are confirmed on the flight and that no ticketing issue exists.