"They do not want to read this bad and long book."
Translation:Они не хотят читать эту плохую и длинную книгу.
Should be accepted, report next time. With the reflective "Им не хочется" this sentence sounds a bit more colloquial.
There is a difference. Your answer means more "They are too lazy to read this book" - more like they would read this book if it wasn't so bad and long.