Navigation button not showing up

I recently spent about a month customizing a template to fit my needs (don’t have any porgrammig knowledge, hence the month). I got most of the things the way I want, and it’s working fine. However, now I noticed that the navigation button ( <– previous posts | next post –>) is not showing up at the bottom of my last post.

Can anyone help me figure out why this is not showing up?

Here’s my blog: https://www.limonphotography.com/blog/

I tried to follow this page, and have tried every one of those codes on my index.php page… and had no luck. Do I have to do something in the style.css file, or am I just not putting something in the right place?