To enable paying through cash, you need to add a cash payment option to the checkout page:
Open the Checkout Layout that you are using on your storefront. (The default is Checkout Layout with GiftCard.)
Switch to grid view.
Add the Payment Gateway widget to the layout.
Add the Cash Payment element to the Payment Gateway widget.
Publish your changes.
Note that when a shopper checks out, the cash payment option appears on the checkout page only if the country being shipped to appears in the list of countries that cash payments are supported for.