Ticketed by KL, operated by DL - not able to select seat after modifying itinerary
 

Hi.

tl;dr — Ticketed by KL, operated by DL. After modifying flight, not able to select a seat via DL's website.

Travel ticketed by KL but operated by DL.

I was originally booked to fly JFK-MXP non-stop on DL, but this morning, I needed to change my plans, so I changed to BOS-CDG-VCE via KL's website. BOS-CDG operated by DL and CDG-VCE operated by AF.

On KL's website, it indicated that the BOS-CDG leg would be in Premium Select. I figured this was just an error in the system because I was originally booked in Main Cabin and the difference in fare was only 20€.

After completing the purchase, KL's website indicated that I was in Main Cabin, just as expected. Also just as expected, they sent me to DL for seat selection.

DL's website, however, is not letting me select a seat, saying "Uh-oh! We're sorry but we are not currently able to assign a seat in your class of service. Seat availability can change, so please check back later. If you do not have a seat assignment, a seat will be assigned at check in or at the gate prior to boarding."
I'm definitely not in Basic Economy/Economy Light, so I'm not sure why DL's not letting me select a seat.

The original fare code was U, but now it's R.
Is this a matter of KL and DL's internal systems not communicating well with each other? Is a call to DL necessary?

Thanks in advance.