This seems to be a continuing issue with the AS codeshares on JL. JL does not recognize the codeshares as being entitled to advance seat selection. As above, try to access the booking at JAL.com (you may or may not be successful). Also see these threads:
https://www.flyertalk.com/forum/alas...selection.html
https://www.flyertalk.com/forum/alas...deshare-2.html

Booking JL operated flights as codeshares with other airlines also caused problems. See for example:
Selecting seats on JAL when booked as AA codeshare

I might try to assign seats using another OW airline website such as RJ, AY, or RAM’s website trying each of the locators (AS, JL) and the flight numbers. This sometimes works for getting seat assignments for other carriers when their respective systems don’t seem to be talking and a couple of people in the AA thread seem to have been successful doing this.