Originally Posted by
WineCountryUA
For LX operated flights you have to talk to LZ. LX does not provide free economy seat assignment. And may not on codeshares
BasicEcon brings a number of additional restrictions -- change fees, check bag fees for non-elites, ....
LX does indeed not allow free seat selection pre-check in on codeshare flights. If it's booked as an LX flight (LX flight number, not UA flight number but operated by LX, for OP), you might be able to pick seats on the LX website.