Compatibility with coupons added on the Order page from wp-admin requires WC3. If you have any further questions, you can woocommerce_coupon_error a new thread. Otherwise, it may interfere with other woocommerce messages. The discount is applied and goes through to the cart without issue.
Payment with discount applied is made.
I just tested this on a test site. The coupon when applied works as expected. Puoi farlo tramite RSS con un lettore di feed.