Resolved Checkout-Payment-Wrong promo code canceled issue

Author: chiragp-wagento

app/code/Magento/Checkout/view/frontend/templates/cart/coupon.phtml

@@ -24,7 +24,7 @@
                 <div class="field">
                     <label for="coupon_code" class="label"><span><?= /* @escapeNotVerified */ __('Enter discount code') ?></span></label>
                     <div class="control">
-                        <input type="text" class="input-text" id="coupon_code" name="coupon_code" value="<?= $block->escapeHtml($block->getCouponCode()) ?>" placeholder="<?= $block->escapeHtml(__('Enter discount code')) ?>" />
+                        <input type="text" class="input-text" id="coupon_code" name="coupon_code" value="<?= $block->escapeHtml($block->getCouponCode()) ?>" placeholder="<?= $block->escapeHtml(__('Enter discount code')) ?>" <?php if (strlen($block->getCouponCode())): ?> disabled="disabled" <?php endif; ?> />
                     </div>
                 </div>
                 <div class="actions-toolbar">

app/code/Magento/SalesRule/view/frontend/web/template/payment/discount.html

@@ -22,12 +22,23 @@
                         <span data-bind="i18n: 'Enter discount code'"></span>
                     </label>
                     <div class="control">
+                        <!-- ko ifnot: isApplied() -->
                         <input class="input-text"
                                type="text"
                                id="discount-code"
                                name="discount_code"
                                data-validate="{'required-entry':true}"
                                data-bind="value: couponCode, attr:{placeholder: $t('Enter discount code')} " />
+                        <!-- /ko -->
+                        <!-- ko if: isApplied() -->
+                        <input class="input-text"
+                               type="text"
+                               id="discount-code-applied"
+                               disabled="disabled"
+                               name="discount_code"
+                               data-validate="{'required-entry':true}"
+                               data-bind="value: couponCode, attr:{placeholder: $t('Enter discount code')} " />
+                        <!-- /ko -->
                     </div>
                 </div>
             </div>