Passing through ICN and getting at least one segment on KE is my bread and butter flying. My friend, you just got hit by the joy of half-baked DL-KE JV. You cannot select seat on KE flights and you cannot use GUC either, DL never tells you this when they advertise the advantage of the JV. I deal with this on every single flight booked on DL that involves a DL-coded KE flight. Those usually have the discounted fare class and KE only allow seat selection on the much higher fare class or its own frequent flyers with their own status.

KE cannot touch the reservation until 24hr before departure, per agreement with DL. DL can select seats but that function on the app and web are broken. You will have to call in and get an experience agent that know how to do seat selection on KE flights that overwrite fare class restrictions. In addition, DL pax usually are placed in the back section of the aircraft, even if you are Diamond, STE+, or MM...i.e., there is a "zone" for codeshare pax and you do not get to select "preferred seats."

However, KE is 100x better than DL, even the average economy seat on KE aircraft is better than DL Comfort+ as far as seat pitch is concerned, so plenty of legroom.