Show a custom message at the checkout when user has unused coupons

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If you want to show a custom message at the checkout for users who have unused coupons they received from referring people, add this code to functions.php file:

add_action('woocommerce_before_checkout_form', 'show_referral_coupons_message');
function show_referral_coupons_message() {

	$userId = get_current_user_id();
	if($userId === 0) {
		return;
	}

	$user_info = get_userdata(get_current_user_id());
	$user_email = $user_info->user_email;
	$args = array(
	    'posts_per_page'   => 99,
	    'post_type'        => 'shop_coupon',
	    'post_status'      => 'publish',
		'meta_query' => array(
			'relation' => 'AND',
		    array(
			  'key' => 'customer_email',
			  'value' => $user_email,
              'compare' => 'LIKE'
            )
		)
	);
    $coupons = get_posts( $args );
    if(empty($coupons)) {
		return;
    }

    $availableCoupons = [];
    foreach($coupons as $coupon) {
        $couponUsage = get_post_meta($coupon->ID,'usage_count', true);
        $usageLimit = get_post_meta($coupon->ID,'usage_limit', true);
        if($couponUsage < $usageLimit || $usageLimit === '') {
            array_push($availableCoupons, $coupon);
        }
    }
    if(!empty($availableCoupons)) {
       wc_print_notice( __('You have unused referral coupons. Add them to get a discount.'), 'notice' );
    }
}