Certain reflexive forms may serve as substitutes for the passive voice. They may often be translated with the words “can be,” although there are also other possible translations:
Dieses Buch liest sich leicht.
This book can be read easily. [or:] This book is easy to read.
Dieser Satz übersetzt sich nicht!
This sentence cannot be translated. [or:] This sentence is untranslatable!
You may find the Review Units 13-16 exercise more valuable now or as a later review opportunity.