If you want to make coupon code as a required field, this plugin will help.
During checkout when a customer will try to place an order without entering any coupon code, checkout process will be interrupted and customer will see an error notice.
You can configure the error notice text from „WooCommerce>Force Coupon“ menu from admin panel.
Dieses Plugin steht zum Download zur Verfügung, um in deiner selbst gehosteten WordPress-Installation verwendet zu werden.