I would like to mark a primary product category

Hello,

Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.

Could you please try using this code for your case here:

/**
 * Show related products from Primary Product Category
 */
add_filter( 'woocommerce_get_related_product_cat_terms', function ( $terms, $product_id ) {

	if ( ! empty( $primary_cats = get_post_meta( $product_id, 'rank_math_primary_product_cat' ) ) ) {
		return $primary_cats;
	}

	return $terms;
}, 10, 2 );

We really look forward to helping you.

Thank you.