By default, customers are able to enter a coupon on both the cart and checkout pages.
Adding the coupon feature to the checkout page adds extra clutter that many sites don’t want. If someone enters a coupon on the cart page, there’s no need to show them the coupon field again at checkout.
PHP
remove_action( 'woocommerce_before_checkout_form', 'woocommerce_checkout_coupon_form', 10 );