No imput with string inside a model

Hi i have a little issue with print a costum field inside invoice.
First i have created a copy of invoices folder in my child theme and then created my template where i put this code

<?php $wpo_wcpdf->custom_field(‘myfield12’, ‘Codice Fiscale/P.IVA:’, true); ?>

For your informations i have installed also this plugin to add new field in checkout page form
https://wordpress.org/plugins/woocommerce-checkout-manager/faq/

The string to print the costum fields is the same of your plugin..maybe can be possibile some conflicts for that??

Please let me know i can send you admin access to investigate

here the link of hte website: http://www.futuracoopsociale.it/emporio/

https://wordpress.org/plugins/woocommerce-pdf-invoices-packing-slips/