I have a confirmed reward reservation with KE (through CO). I went to KE.com to change my seating assignments, but the Select Seats button is not clickable. When I rolled over it, it said something like it is available within 90 days of the flights. My flights are in early December (in about 30 days). What did I do wrong?

It may depend on exactly how your reservation was made. I flew on an award ticket issued by DL for KE flights. In that instance, IIRC, I took care of my Business Class seat assignments over the phone with KE. (Those were the days before advance seat selection through KE.com.) But when I book paid tickets through DL as DL codeshare flights, KE can't do anything about the seat assignments and DL must do it.

So if you've got CO-coded tix, you might need to call CO. That's about all I can offer.