This issue reports pages that do not receive the return link from all pages specified in hreflang annotations. If page A links to page B, page B must link back to page A.
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Example 1:

Hreflang annotations on page A (https://ahrefs.com/blog/website-audit/):

Hreflang annotations on page B (https://ahrefs.com/blog/zh/website-audit/):

As you can see, page B does not link back to the english version (page A) from its hreflang annotations.

Example 2:

Hreflang annotations on page A:

Hreflang annotations on page B:

In this example, the optional "x-default" hreflang value is used on page A, but not on page B, which will also trigger this issue in Site Audit.
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Several more cases:
1) More than one URL in the group is marked as x-default

2) Hreflang tag used in the page code doesn’t match the one specified in the sitemap

3) Language used in the hreflang tag in the page code doesn’t match the language specified in the sitemap.

If the return tag is missing, those annotations may be ignored or misinterpreted by the search engines.

Review all the pages listed in this report. To fix the issue, make sure all the page versions translated or targeted to users in a certain region have the exact same set of hreflang annotations which includes URLs to all page versions. This will always provide return links between the alternate pages.

Google's guidelines on hreflang implementation