coupons not working between cart and checkout in blocks
On the new block-based cart & checkout, if a coupon code is entered, the discount doesn't get passed between the cart and checkout, so you have to delete the coupon and re-add it if you've added it at the cart, or similarly if you add it at the checkout and then return to the cart.

We cannot reproduce this issue – would you be able to please log this as a bug to our repo: https://github.com/woocommerce/woocommerce-gutenberg-products-block and provide more details so we can try assist you.
-
Mark commented
The current adding of a coupon doesn't work in version 2.7.1. When the coupon is added and it has a discount, the discount is not being applied to the price. I am using Wordpress 5.4, woocommerce 4.2 and php 7.4