Libre Office used to print 2 sided. After the last few months the only way I can print 2 sided is to export to .pdf and print that. There is nothing in the former posts that works and most of the go to this or that is no longer there in this version of LibreOffice.
Printer everything is set to do 2-sided copies. LibreOffice will not work.
Maybe the link in this answer (not marked as solved) will help, Writer will not print duplex (2-sided) pages despite proper printer setting and LO print setting - #8 by mmokrejs
Note that 6.1.5.2 is quite old now and will not have any bug fixes released since then.