1, Go to Modules -> Module Manager.

2, Search Cross-selling  and click on configure button.

3, Fill required fields.

PRESTASHOP 1.7.X – HOW TO MANAGE “PRODUCTS IN THE SAME CATEGORY” MODULE #

https://www.templatetrip.com/help/prestashop-1-7-x-how-to-manage-products-in-the-same-category-module/

PRESTASHOP 1.7.X – HOW TO CREATE A RELATED PRODUCT #

https://www.templatetrip.com/help/prestashop-1-7-x-how-to-create-a-related-product/