kco-display-error-notice.php

<?php
// Display an error notice abouve the KCO checkout if the customer is under 18 years.
add_action('klarna_before_kco_checkout', 'my_klarna_before_kco_checkout' );
function my_klarna_before_kco_checkout() {
 	if ( isset( $_GET['under_age'] ) ) {
		echo '<div class="woocommerce-error">';
		_e( 'You need to be over 18 years to make a purchase.', 'woocommerce-gateway-klarna' );
		echo '</div>';
	}
}