title: How can I avoid 'A product is missing a field: google_product_category' warning in facebook microdata debugger tool?
body: <p>The tool 
  <a href="https://business.facebook.com/ads/microdata/debug" rel="nofollow noreferrer">https://business.facebook.com/ads/microdata/debug</a>
  gives me the warning</p>
  
  <p><a href="https://i.stack.imgur.com/ApjKQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ApjKQ.png" alt="A product is missing a field: google_product_category            Without google_product_category information, your products may not appear the way you want them to in ads."></a></p>
  
  <p>Which OpenGraph tag can I use to fix the problem?</p>
  
  <p>The HTML code like </p>
  
  <pre><code>&lt;meta property="google:product:category" content="Pet Supplies"/&gt;
      or
  &lt;meta property="google_product_category" content="Pet Supplies"/&gt;
  </code></pre>
  
  <p>doesn't work for me.</p>