If you have posts that are related, perhaps the issue is that YARPP doesn’t consider them related “enough.” You can fix this by going to the YARPP settings page, clicking on “Screen options” in the upper right, and checking the box for “Relatedness” options.” You will then be able to select a lower match threshold to make more posts show up as related.
If you don’t yet have any posts that are related to each other, “no related posts” is what will show. You can fix this by going into settings, and deleting everything in the box that says, “Default display if no results:”. That way, related posts will show once you have them, but nothing will show for a post that doesn’t have related posts.