Can you post a link to the file?

I can do some testing here.

-Mike

I was able to successfully verify the media file URL within my test site. (You can see a screenshot of that here: https://content.blubrry.com/blubrry_support_docs/MediaURL.png)

There may be a conflict happening within your WordPress installation. Make sure core WordPress, all plugins, and your WordPress theme are up to date. This support document also has suggestions on how to diagnose conflicts within your WordPress installation:​
https://blubrry.com/support/powerpress-documentation/diagnosing-feed-andor-player-issues-with-powerpress/​

Also, keep in mind it isn’t mandatory for PowerPress to verify media file URLs. If you know the URL is valid and working normally, you can just go ahead and publish the post and PowerPress will add the post and media file URL to your podcast RSS feed.