There is no explanation in the book that indentation is significant and needed for the compiler. When searching for 'Indentation' it is only referenced in the Style Guide, Migration Guide and Language Reference.
I think for new learners that start learning Scala with Scala3 and the Scala3 book, it would be very helpful to add a few lines of explanation e.g. in A Taste of Scala - Control Structures or even a chapter of its own, that indentation is significant and must be adhered to.