WooCommerce - Allow coupons to be usable by existing customers only

functions.php

<?php
// Made by Goran from https://wpgens.com 
add_action( 'woocommerce_before_apply_coupon', 'check_order_history_before_coupon' );

function check_order_history_before_coupon( $coupon_code ) {
    // Assuming 'firstorder' is your coupon code
    if ( strtolower( $coupon_code ) !== 'firstorder' ) {
        return;
    }

    $user = wp_get_current_user();
    $email = '';
    $has_orders = false;

    if ( $user->ID ) {
        // For logged-in users
        $orders = wc_get_orders( array(
            'customer_id' => $user->ID,
            'status' => 'completed'
        ));
        $has_orders = count( $orders ) > 0;
    } else {
        // For guests, check if there's an email in the current session
        $email = WC()->session->get('customer')['email'] ?? '';
        if ( $email ) {
            $orders = wc_get_orders( array(
                'billing_email' => $email,
                'status' => 'completed'
            ));
            $has_orders = count( $orders ) > 0;
        }
    }

    if ( !$has_orders ) {
        wc_add_notice( 'This coupon is only for customers who have already made an order.', 'error' );
    }
}