Agent no is the id number for the pickup point chosen by your customer.
You will get this error when you are trying to create a label to a pickup point there is no longer valid.
This usually happens if the parcel shop has been removed from the carriers system.
You can change the pickup point / Agent no. in WooCommerce, PrestaShop and Magento.
In Shopify, it is unfortunately not possible to change the shipping method after the order has been placed.
In Shopify you can there either duplicate the order and use one of the standard Shopify Backend Methods or your can create the order manually from your carrier's platform.