The best way to handle seat assignment on an award flight reservation on LH, is to call LH, give them your eticket number and ask them for the LH record locator. Then you can select your seat online on the LH website (and to OLCI as well, which is useful in the case you have *G status as exit rows in Y become available at check-in).

FWIW, I first tried to call UA for the LH record locator, but they provided me with a completely wrong one... Fortunately a quick call to LH (the USA reservation number has very little or no wait) provided the correct record locator and assigned 83C to Ms. CASAFlyer