How to Use Related Posts Area
Related Posts area works based on post tags.
Please be sure if you tagged your posts by using native Wordpress tag tool.
If you want to use this area properly your posts must be tagged, theme shows related posts based on the relation of tags of each posts and recommends some random posts for each post.
If you have already tagged your posts and related posts is invisible, please be sure it is not hidden in Wordpress Customizer.