Problema al agregar NIF en factura WooCommerce

// Incluir NIF en la factura

add_filter( 'wpo_wcpdf_billing_address', 'incluir_nif_en_factura' );

function incluir_nif_en_factura( $address ){
  global $wpo_wcpdf;

  echo $address . '<p>';
  $wpo_wcpdf->custom_field( 'NIF', 'NIF: ' );
  echo '</p>';
}

add_action( 'woocommerce_after_order_notes', 'wpdesk_vat_field' );
/**
* NIF en WooCommerce Checkout
*/
function wpdesk_vat_field( $checkout ) {
    echo '<div id="wpdesk_vat_field"><h2>' . __('NIF') . '</h2>';
    
    woocommerce_form_field( 'vat_number', array(
        'type'          => 'text',
        'class'         => array( 'vat-number-field form-row-wide') ,
        'label'         => __( 'NIF' ),
        'placeholder'   => __( 'Introduce tu NIF' ),
    ), $checkout->get_value( 'vat_number' ));
    
    echo '</div>';
}

add_action( 'woocommerce_checkout_update_order_meta', 'wpdesk_checkout_vat_number_update_order_meta' );
/**
* Guardar NIF en order meta
*/
function wpdesk_checkout_vat_number_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['vat_number'] ) ) {
        update_post_meta( $order_id, '_vat_number', sanitize_text_field( $_POST['vat_number'] ) );
    }
}

add_action( 'woocommerce_admin_order_data_after_billing_address', 'wpdesk_vat_number_display_admin_order_meta', 10, 1 );
/**
 * Mostrar NIF en la sección de editar pedido
 */
function wpdesk_vat_number_display_admin_order_meta( $order ) {
    echo '<p><strong>' . __( 'NIF', 'woocommerce' ) . ':</strong> ' . get_post_meta( $order->id, '_vat_number', true ) . '</p>';
}

add_filter( 'woocommerce_email_order_meta_keys', 'wpdesk_vat_number_display_email' );
/**
* NIF en Emails*/
function wpdesk_vat_number_display_email( $keys ) {
     $keys['NIF'] = '_vat_number';
     return $keys;
}