Lufthansa began charging for advance seat reservations for lower discounted economy fares earlier this year.
Lufthansa began charging for advance seat reservations for lower discounted economy fares earlier this year.And apparently, it can only charge for the seat reservations if the ticket is issued by Lufthansa (ticket number beginning with 220).
The wiki at the top of the ft thread says that UA can't process payment for the advance-seat-res fee.
Apparently, you can't pay that fee directly to LH because UA issued the ticket.
This was not an issue with LH and other partners until Lufthansa began charging for seat reservations for tickets issued beginning April 28 of this year.
Not everything in that FT thread is relevant to your situation, but it does include some who were similarly surprised after buying tickets.
I've flown code shares before (on other airlines) and have never run into this.