I have a Europe-US flight booked via Expedia on DL metal but with the return TATL leg a KL flight number.

I am able to use DL.com to change my seat on all DL flight numbers but the KL leg gives me the following message:

'We are sorry there are no seats currently available for advanced seat selection. Please check back later or receive your seat assignment at the gate prior to boarding'

Is there anyway I can get past this? Currently I am assigned a row right at the back with many free seats available