For every "<enclosure> tag" there must be a type attribute called  type="audio/mpeg" and a length.   

This is an example of a valid item tag:
 <item>
      <title>Title </title>
      <guid isPermaLink="false">5a0d7e389149d315ac46329e</guid>
      <description>Description</description>
      <category>SOME_CATEGORY</category>
      <pubDate>Thu, 16 Nov 2017 12:02:00 GMT</pubDate>
​      <enclosure> url=http://www.nobexpartners.com/ODItem.aspx?id=XXXXXXXXXX.mp3" type="audio/mpeg" length="60099756" />
      <itunes:subtitle>SUBTITLE</itunes:subtitle>
      <itunes:summary>SUMMARY</itunes:summary>
      <itunes:image href="http://www.nobexpartners.com/ODItem.aspx?id=XXXXXX.i.jpg" />
      <media:content url="http://www.nobexpartners.com/ODItem.aspx?id=XXXXXX.mp3" fileSize="60099756" duration="2507" type="audio/mpeg" />
    </item>

Happy Podcasting!