Hi @marc-bovetgmail-com
Because WooCommerce is the one that handles the checkout page, ACF won’t be able to do anything on that page.
To add ACF input fields, you can use the acf_form() with ‘form’ option set to false. That way it will be integrated to the checkout form. But you need process this form manually.
On the link I gave you before, it uses woocommerce_checkout_update_order_meta hook to process the form, so you need to do it here too. Using this hook, you can use the update_field() function to update the field for the order.
I hope this makes sense 🙂
