Hi Kate,
This appears to be an issue with the plugin. It was working on a site that I manage, and it stopped a few days ago.
Here are the errors I receive:
<aside id="aptfpinbytap_domain-2" class="pinterest"><div class="pinterest-title"><span style="background-color:#fff; padding-left:5px;">Featured Pinterest Gallery - What is Beauty?</span></div><!-- Using AlpinePT for Pinterest v1.2.6 with Pinterest RSS --><!-- Request made --><!-- Failed using wp_remote_get(), simplexml_load_string() and XML @ https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rss --><!-- Failed using simplexml_load_file() and XML @ https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rss --><!-- Failed once using fetch_feed() and RSS @ https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rss --><!-- WP Error message: A feed could not be found at https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rss. A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed. --><!-- Try SimplePie --><!-- Failed twice using fetch_feed() and RSS @ https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rss --><!-- SimplePie Error message: This XML document is invalid, likely due to invalid characters. XML error: XML_ERR_NAME_REQUIRED at line 28, column 414 --><!-- Sorry:<br>- Please recheck your ID.--></aside>
It appears as though Pinterest no longer uses those RSS URLs. When I attempt to load https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rss in the browser, I receive a 404.
However, loading https://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty/rsshttps://pinterest.com/colorstylstudio/colorstylestudio-what-is-beauty.rss does work.
Unfortunately, not much can be done until the plugin authors update the plugin to work with the correct Pinterest RSS URLs.