Hey all!

I'm flying Denver to Sweden via Frankfurt in September and have roughly 75k miles and more sitting in my account that I never utilize. I'd like to upgrade my flight either to business class one way or premium economy round trip. I booked the basic economy fair and confirmed at booking that they were both "upgrade with miles eligible". Now when I go the booking and go through the upgrade process, it shows availability via cash but not via miles. It simply states "the upgrade booking class (premium economy/business class) is not available for this flight".

I'm in class "V" outbound and "S" inbound, all on LH metal.

Any ideas on why I can't utilize miles?

Thanks all!