Related posts are “connected” via tags. In order for a post to show as “related”, it should share a common tag with the post that the user is currently viewing at the front-end. Try adding the same tag for both posts, and it should work correctly.
Related Posts Settings at the Customizer
Of course, double check that the related posts settings are correct under
Appearance > Customizer > Layout: Posts, like the screenshot below shows. You can choose to show or hide the whole section, change the title to whatever you wish, and select how many related posts you want to show.