Hello,
01. In order to display related posts, you need to create tags and then put these tags into the posts. The posts having equal tags will be displayed as Related posts.” now I have added the tags on your given post and the related posts are showing fine. Check back your site and check the issue.
02. Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site and then check the issue. I am sure your issue will be solved. Then Activate the 3rd party plugins one by one and check which plugin is creating the issue for you.
Best Regards,