How do I print double sided?

If you believe you have the correct driver already installed, it may be time to reset the printing system on you Mac.

Basically, you're just going to delete the current printer(s) & scanner(s) and reinstall them.

Here's more info about that:

If you can't print from your Mac or iOS device - Apple Support

You install the correct print drivers for your model printer if it supports double sided printing.