In order to add cash on delivery to your PrestaShop 1.7 store go to the Improve section, select the Modules and then Catalog. In the search field type "cash" and confirm with enter. From the list of available modules, select "cash on delivery" and click install.
The next step is to create a carrier. In the Improve section, click Shipping and then Carriers. Read more about adding carriers in your PrestaShop store. Create a new Courier-Delivery form by clicking +add new carrier.
The last step is to define the available payment methods in your PrestaShop. In the Improve section go to Payment, Shop Parameters. From the list select the newly created carrier and check the available options.