Problem
The original product review HTML code is not using Google Review Snippet format
Solution
Replace /Snippets/ProductReviewItem.html with below code (apply to both Desktop and Mobile templates)
<li itemscope itemtype="https://schema.org/Review"> <span itemprop="itemReviewed" itemscope itemtype="https://schema.org/Product"> <meta itemprop="name" content="%%GLOBAL_ProductName%%"> <meta itemprop="url" content="%%GLOBAL_CurrentProductLink%%"> </span> <h4> <span itemprop="name">%%GLOBAL_ReviewTitle%%</span> <span itemprop="reviewRating" itemscope itemtype="https://schema.org/Rating"> <img width="64" height="12" src="%%GLOBAL_IMG_PATH%%/IcoRating%%GLOBAL_ReviewRating%%.png"> <meta itemprop="ratingValue" content="%%GLOBAL_ReviewRating%%"> <meta itemprop="bestRating" content="5"> </span> </h4> <p style="color:grey"> <span itemprop="datePublished">%%GLOBAL_ReviewDate%%</span> by <span itemprop="author" itemscope itemtype="https://schema.org/Person"> <span itemprop="name">%%GLOBAL_ReviewName%%</span> </span> </p> <p>"<span itemprop="reviewBody">%%GLOBAL_ReviewText%%</span>"</p> <div itemprop="publisher" itemscope itemtype="https://schema.org/Organization"> <meta itemprop="name" content="%%GLOBAL_StoreName%%"> </div> <hr> </li>
Finally go to https://search.google.com/structured-data/testing-tool to confirm the result

Reference
Google has changed its schema, the testing tool unfortunately errors as below.
One of offers or review or aggregateRating should be provided.
Is there a fix for this?
Let me take a look, will update you soon.