Riflifrupp49 Hello
From Milano to Bergamo you have a local train and you must make a check-in before boarding the train to get a QR code. You should find a link in the confirmation mail to make this check-in or you can import your ticket in the Trenitalia app with its number and your e-mail address and make the check-in in the app. It you take another train, change the train before the check-in.