If you choose "Viewed Together" or "Bought Together" Data source configuration, the related product will be shown according to views/sales history. The "Product Currently" viewed option allows you to promote products without the views/sales history.
There is functionality to show cross-cells/up-cells/related products for "Related Product Rules". At first, please change in Data Source "Product Attribute" to "Related Product Rules" >>
After that you should create a new rule according to the steps in our guide.