Next, Previous links not showing up

I’m assuming this is a common problem and that there may be multiple reasons why it doesn’t work. Can anyone point me in the right direction to do some troubleshooting?

You can view a page here: https://estevancarlos.com/v7/projects/little-doll-and-monk/

I’m trying to use this: <?php posts_nav_link(); ?>

Nothing shows up. Suggestions?