If you have a duplex capable printer that does not give you the option to duplex when you go to print.
One thing to check is that the driver is configured properly. On your computer, go to control panel>devices and printers and then right click on the icon for your printer. Select "printer properties" from the drop down menu. Click on the advanced tab and ensure that you have an HP driver installed not a "Class or IPP driver" installed. If you do not have an HP driver listed, delete the printer and start over. If it is an HP driver, next click on the device settings tab. Scroll down and make sure that under installable options it shows that the duplex is installed. Changed to installed if it is not.