There is no mandatory minimum and / or maximum value for the assessment.

Hello,

I couldn’t log in to your site with the details you added in the Sensitive Data Section. Can you please check and add the correct username and password?

Looking forward to helping you. Thank you.

Hello,

I see 1 review is missing the rating on your site: https://i.rankmath.com/7DQTAv. I am not sure, how you removed the ratings as it’s a required field and user won’t be able to add a review without adding a rating. This is not the Rank Math plugin issue, even with WooCommerce snippet you’ll get the same result.

For now, to remove the null rating review, please add below filter code in your theme’s functions.php:

add_filter( 'rank_math/snippet/rich_snippet_product_entity', function( $entity ) {
	foreach ( $entity['review'] as $key => $review ) {
		if ( empty( $review['reviewRating']['ratingValue'] ) ) {
			unset( $entity['review'][$key] );
		}
	}
	// $entity['brand'] = 'Rank Math';
	return $entity;
});

This should fix the schema = error but you should check how user was able to add a review without selecting a value for rating.

Hope that helps. Thank you.