set "Modules under the product page"，how to enable "Related products"?

Hi,

In the "modules under the product page" setting, you need to search for the "related products" module, select it and then save the configuration and you'll be good. It should actually already be preselected.
If you don't have the "related products" module in that setting, it's probably that this module was deleted for some reason. In that case, you can go in the Joomla modules manager, click on the "new" button, select the "HikaShop content module" type and configure it liek the default related products module and then select that module in the "modules under the product page" setting.