Your post-checkout product recommendations block will not start popping up for your customers to see if you have not selected the products to recommend in it.
The post-checkout upsell recommendations rely on the functionality of the add-on Related Products. For example, if you have products A and B and want Product B to be shown in the post-checkout product recommendations block after someone buys Product A, you need to specify Product B as related to Product A. This way, when the block is displayed after your customer completes checkout for Product A, it will automatically provide a recommendation for Product B.
To specify a product as related to another product, follow the steps below:
In your store's Admin area, open the details of the product buying which you want to trigger your store's post-checkout upsell behavior (Product A). Then, switch to the Related Products tab of the product details.
In the Related Products section, click Add related products.
Select the product you want to be shown in the post-checkout product recommendations block when someone buys Product A from the list. It will be your related product for Product A. (In our example, it will be your Product B). If you want more than one product recommendation, select several products. Your online store will mark all chosen products as related to Product A.
Further below, the screenshot will demonstrate two products selected for adding as related.
After selecting the products you require, click Add products to save your selection.
Note that if you want the relationship between Product A and its related products to work in both directions, you will need to enable the "Make bidirectional relations" checkbox before saving:
This way, you will not have to go through all the above steps again to specify that Product A is related to Product B.
Once you click Add products, the selected products will be recorded as related to the original product. You will be able to view them as a list at any time via the Related Products tab in the product details.
Note that the Related Products tab provides you with all the necessary controls to manage the related products. For example, every related product on the list has a Trash icon that enables you to delete the product relationship, so your online store will no longer display the product in the post-checkout product recommendations block. In addition, some "four-headed arrow" controls enable you to rearrange the products on the list to specify the priority in which they need to appear in the post-checkout product recommendations block. And, of course, there is the "Add related products" button allowing you to add more related products.