Hi,
As the documentation says and as Mohamed told you, in the configuration of HikaShop, click on the "Select" button of the "Modules under the product page" option. Set your related module to published and click on OK. Then, save the configuration.
You didn't provide any screenshot of the configuration showing that you had added the related products module to the "Modules under the product page" option of the configuration.
I suppose that it's probably what you're missing.
In any case, if you use a recent version of HikaShop, the related products module is already installed by default and already setup to be displayed under the product page. All you need to do is to add products as related products in your products.