While it’s possible to add a payment plan during the checkout process on the Cart page, it is not possible to do this directly on the Checkout page.

However, if you have sufficient experience in WooCommerce programming, it is possible to customize the plugin to add this functionality. Here are the basic steps that you would have to follow:

1. Find the woocommerce-payment-plans-for-subscriptions/public/class-wc-payment-plans-public.php file and the function within it called cart_item_name_extra(), which adds the list of available payment plans to the Cart page.
2. Copy the cart_item_name_extra() code and paste it into the woocommerce/checkout/review-order.php file that resides in your active theme. If you don’t have this file in your theme, you can copy it from the woocommerce-payment-plans-for-subscriptions/templates/checkout folder.
3. Replace the line is_cart() with is_checkout() and save the file.
4. Add JavaScript code to refresh the cart on the Checkout page during an on_click() event. You can find similar code in the woocommerce-payment-plans-for-subscriptions/public/js/scripts.js file.

NOTE: Making this change is only for experienced developers. We cannot provide any support for plugin customization. If issues arise, please review WooCommerce Developers Resources.