Aeroflot—can't select seats on partner flight?
 

I booked a flight from Incheon to New York through Aeroflot in January with a layover in Moscow. The first leg of the trip is operated by Korean Air.

I paid extra in order to select my seats, as did my friend with whom I'm traveling, but we're unable to select seats for this first flight so that we can sit together. Aeroflot says that they're unable to open the system for a Korean Air flight, and Korean Air claims that they can't modify our reservation since it's booked through another airline. (We have the confirmation numbers appropriate to both airlines.)

Any thoughts on which airline would be able to allow us to select seats, and how to go about this? Thank you!!

The marketing carrier would normally be responsible for advance seat reservations. In this case SU.

Code shares are always tricky, and sometimes seat selection will only be available during OLCI...
I don't know about SU marketed and KE operated, but SU marketed and KL operated is one of those examples.

If both airlines claims they cannot do anything, you should HUCA, if you get the same answer still, you are probably out of luck (and ask for any seat selection fees you might have paid to be refunded)