You have 3 conditions to show related products: http://prntscr.com/ik7p9p
All product must be the same collection
+ If you choose “Handle”, you need to enter the handle of the main product as a tag from the related product.
For example, the Main product is https://demo-basel2.myshopify.com/products/backpack-double-strap-1 and I want Basic contrast sneakers is the related product. So from Basic contrast sneakers page, I will enter tag backpack-double-strap-1 http://prntscr.com/ik7td5 and it will show http://prntscr.com/ik7u4j
+ If you choose “Tag” so all product have 2 tags are same, example tag 1, tag 2 in both product A and B so they are the related product