Same thing here, but with the previous version of the plugin (https://es.ww.wp.xz.cn/plugins/book-review/).
I have changed the source code (a minor change) from “https://schema.org/Thing” to “https://schema.org/Book”.
It seems that google is not accepting the rating on generic things:
(spanish)
https://support.google.com/webmasters/thread/28377569?hl=es
I this change gets accepted, I will let you know.
