Hi Rusyl,

Thank you for your message.

This setting will look for the actual checkout page from the WooCommerce plugin. You can set this up through these steps:

1. Create a new blank page called "Checkout"
2. Add the following shortcode (copy/paste) to that page: [woocommerce_checkout]
3. Save the page
4. Go to WooCommerce -> Settings -> Advanced
5. At the "Checkout page" setting select the page you created at step 1
6. Save the settings
7. Go to Appointments -> Settings -> WooCommerce
8. At the "Redirect after booking" setting select "Checkout page"
9. Save the settings

Now when you make a booking you will be redirected to the checkout page. 

Hope this helps.

Under the "Products" tab on your dashboard you can add a product (or add a service) and while creating the product you can enable the "Booked Product" checkbox (see 1st attached image).

When a product have this Booked Product checkbox enabled you can select it at the Booked custom field settings (see 2nd attached image),

Hope this helps.