The payment system is connected and all quick payment buttons in the carts are also enabled.
I take it you’re not referring to the Buy Now buttons within the design page but just in case, it’s enabled in the settings of the product page. Similarly, you can set the Checkout button on the cart for quick checkout.
There’s also the Fast Checkout but a member area needs to be added to the site. This allows users to save their account using their saved details: About Fast Checkout | Help Center | Wix.com
If you mean more along the lines of say Apple Pay/Google Pay for a third-party payment processor, you may need to enable it within the payment processor’s dashboard.