What Does it Mean?
Pages of the site link to different URLs in the rel=canonical tag and the Open Graph URL tag.
Open Graph is a standard that provides rich meta data about a page frequently used to create "cards" about a page when a link is shared on social media or messaging platforms. Although the standard was originally created by Facebook (now maintained by the Open Web Foundation), the cards are used by many other platforms, including Twitter, LinkedIn, iMessage, and Slack. These cards may contain an image, description of the page, a site name, and other data that is much more visually appealing than just a simple link.
Open Graph tags are <meta> tags with the property attribute set to a list of supported types with the prefix "og:". Here's an example of the "title" Open Graph tag:
<meta property=”og:type” content=”website” />
There are four required Open Graph tags: "og:title", "og:type", "og:image" and "og:url". Of these, the "og:url" tag should link to the canonical version of this page. The issue is spotted when the URLs don't match between the rel=canonical target and the Open Graph URL tag.
Why It's Important
Ensuring the URL Open Graph tag links to the canonical URL will consolidate all non-canonical pages to this one as the permanent ID in the graph, which helps keep things simple and structured.
Potentially more important than this is that this issue can frequently point to the existence of other canonicalization issues. It could be possible that the wrong URL is being used in the rel=canonical field.
How To Fix
Decide which URLs (if any) should be the canonical URL for the page. Then update either rel=canonical or Open Graph URL tags that don't match this correct canonical URL.