Before adding highlights, you need to register the book.
Go to Bookshelf and tap “Add Your First Book.” This will open the camera so you can scan the book’s barcode. If scanning does not work, you can manually enter the ISBN code from the top-left corner of the scan screen.

Tap “To Bookshelf”

Tap “Add Your First Book”

The scan screen
Once the ISBN is entered, the rest of the book information will be filled in automatically.
Tap Add at the bottom right of the screen to complete the book registration.

Check the book info and tap “Add”
Video How to Register a Book