[SOLVED] "Thing is not a known valid target type for the itemReviewed property" in Google's Structured Data Testing Tool for Rich Snippet Schema

Since Google's September 2019 update, you may see this error when trying to validate your review schema; "Thing is not a known valid target type for the itemReviewed property"

Can you still use Thing as a target type for valid schema? Yes.

But don't - At least if possible, as you'll find that your review stars and associated schema are dropping from Google's SERPs, and that's no good.

This issue will affect developers who have manually added rich snippet schema for reviews as well as those using rich snippet plugins (or themes with native rich snippet support) that have yet to be updated with this change in mind.

According to Google this is because;

Search results that are enhanced by review rich results can be extremely helpful when searching for products or services (the scores and/or “stars” you sometimes see alongside search results).

To make them more helpful and meaningful, we are now introducing algorithmic updates to reviews in rich results. This also addresses some of the invalid or misleading implementations webmasters have flagged to us.

Making Review Rich Results more helpful | Google Webmaster Central Blog

Focus on schema types that lend themselves to reviews
While, technically, you can attach review markup to any schema type, for many types displaying star reviews does not add much value for the user.

With this change, we’re limiting the pool of schema types that can potentially trigger review rich results in search. Specifically, we’ll only display reviews with those types (and their respective subtypes):

Making Review Rich Results more helpful | Google Webmaster Central Blog

So what's the fix?

In order not to have your review pages flag for "thing is not a known valid target type for the itemReviewed property", you'll need to change the target type from Thing for the itemReviewed to one of the following:

schema.org/Book
schema.org/Course
schema.org/CreativeWorkSeason
schema.org/CreativeWorkSeries
schema.org/Episode
schema.org/Event
schema.org/Game
schema.org/HowTo
schema.org/LocalBusiness
schema.org/MediaObject
schema.org/Movie
schema.org/MusicPlaylist
schema.org/MusicRecording
schema.org/Organization
schema.org/Product
schema.org/Recipe
schema.org/SoftwareApplication

Full explanation from Google here.

Bonus: For RankMath plugin users, there's a quick fix until they roll out an update. Just pop this snippet into your theme's functions.php to override the Thing value to Product (anywhere after the opening <?php tag):

add_filter( 'rank_math/snippet/rich_snippet_review_entity', function( $entity ) {
    $entity['itemReviewed']['@type'] = 'Product';
    return $entity;
});