Even to link a product to the checkout page there should be something in the cart.
Here is What you can do to achieve your goal
1)Add the “add to cart” button on the product page (it is visible by default). You can change it’s name to “Checkout” to create the illusion for users. (from plugin’s settings)
2)select the “skip cart page ” option in plugin settings.
3)Follow Given image’s point no 5 to connect checkout page
This eventually will follow the whole process and connect to the checkout page.
But if even then you face any issue you can share your website details with us. We will try to help you with this.