Originally Posted by tulipan

What booking class did you book your flight in? If you booked Basic Economy then that will explain why you can’t select Comfort+ (not even for pay).
Tiny letters on the Flying blue site:
Flying Blue Silver, Gold and Platinum members with Delta Main cabin fares in eligible booking classes benefit from discount on Delta Comfort+® seats, subject to availability. All fares are eligible except Basic Economy fares (booking class E), which are not eligible for free or paid Delta Comfort+®, regardless of Flying Blue status.