USB Main L/R of the 64S are not routed anwhere, because hat 64 inputs (sends) occupy all 64 USB streams. As there probably are not many cases of 64 track recording, you can simply use any of the non-recorded USB streams for Main L/R. Not sure if the 64S song template offers a setting, but USB 63 and 64 might be hot candidates for the job... ;-)
StudioLive_Series_III_OwnersManual_V6_EN_10062019, section 14.2.4 USB Sends