Enable Duplex (2-Sided) Printing for Macs
How to enable duplex printing for most printers on MacOS.
Click on the Apple icon in the upper-left and select "System Preferences...".
Select "Printers & Scanners" from the System Preferences menu.
Select your printer from the list on the left. Then click "Options & Supplies...".
Select the "Options" tab at the top. Then check the "Duplex Printing Unit" option.
Click "OK". You should now be able to select 2-sided when printing.