Add EU VAT in Invoice PDF Packing Slips

Hello,
thank you very much for the plugin,
I am trying to include the EU VAT field in the invoice with this code but it does not wotk, I can’t collect the value of $customer_eu_vat_number

/**
Include VAT in invoice (WooCommerce PDF Invoices & Packing Slips plugin required)
*/
 
add_filter( 'wpo_wcpdf_billing_address', 'include_vat_invoice' );
function include_vat_invoice( $vat ){
  global $wpo_wcpdf;
 
  $customer_eu_vat_number = get_post_meta( $order_id, '_' . alg_wc_eu_vat_get_field_id(), true );
  
  echo $vat . '<p>';
  $wpo_wcpdf->custom_field( $customer_eu_vat_number, 'EU VAT: ' );
  echo '</p>';
}

any help would be welcome,
thank you!!