$taxonomy = 'product_cat'; 
   $primary_cat_id=get_post_meta($product->id,'_yoast_wpseo_primary_' . $taxonomy, true);
    if($primary_cat_id){
       $primary_cat = get_term($primary_cat_id, $taxonomy);
       if(isset($primary_cat->name)) 
           echo $primary_cat->name;
    }