Customers cannot track their parcels
In order to enable the tracking code to be sent to Shopify, you must ship ( PROCESS button) your orders in the application's back office and not from Shopify.
By default, Shopify does not include the tracking number in emails sent to customers to confirm shipment. It is therefore up to you (or your developer) to modify the emails so that the tracking number appears directly in the Shopify transactional email.