You have not added a QR CODE at the TEXTBOOK level(NOT at the TEXTBOOK UNIT level) 

If you enter the QR CODE  at the TEXTBOOK level,  textbook can be easily accessed using the QRCOde scanner. . Generally it is a good practise to add the QRCOde at the Textbook level .

Click on “Edit Details” and  add  QRCode in the  QR Code field