What Does "Non-reciprocal hreflang tag" Mean?
The URLs have an hreflang tag on them, but are missing one or more reciprocal links back this URL.
Hreflang is a way to tell search engines that there are multiple versions of a page in different languages. This will help search engines serve visitors the most appropriate version of each URL by language or region.
Here's an example of hreflang tags for https://example.com/en-us/products, which is an English page targeting visitors in the United States that also has an English version targeting United Kingdom, a French version, a German version, and a default version available:
<link rel="alternate" hreflang="en-us" href="https://example.com/en-us/products" />
<link rel="alternate" hreflang="en-gb" href="https://example.com/en-gb/products" />
<link rel="alternate" hreflang="fr" href="https://example.com/fr/products" />
<link rel="alternate" hreflang="de" href="https://example.com/de/products" />
<link rel="alternate" hreflang="x-default" href="https://example.com/products" />
In order for hreflang tags to work properly, links must be reciprocal. Meaning, if page A links to page B, then page B needs to link back to page A.
In the above example, each of the URLs linked here should also have an hreflang tag to https://example.com/en-us/products, with the value set to "en-us". If any of them do not have this link, then this issue will be flagged for https://example.com/en-us/products.
Why It's Important
If hreflang links are not reciprocal, they may be ignored or misinterpreted. If they're ignored, it's as if there are no hreflang links at all. If they are misinterpreted, it could lead to a situation even worse than having no hreflang tags at all.
Therefore, this is a very important issue to fix to make sure the correct language content is served to the right audience.
How To Fix
Look at the hreflang links on each page and visit each of these URLs. Ensure that all URLs in this group have the same set of hreflang tags on them. This way you can be certain that all of them have reciprocal links. If any are missing, add the appropriate hreflang tag to this page.