Missing self-reference hreflang annotation
This means that the URL in question has no self-referencing hreflang annotation.
Why is this important?
Self-referencing hreflang is not actually a mandatory requirement. It appeared in the original recommendations that Google published as they figured it would be easier for users to just copy/paste the same 'hreflang cluster' on all corresponding translation pages.
So it is NOT a problem if your site does not use it.
However, some website owners find it easier to manage hreflang when it does include self-referenced annotations.
What does the Hint check?
This Hint will trigger for any URL which has no self-referenced hreflang annotation, using any method (HTML, HTTP header, or XML Sitemap).
Examples that trigger this Hint:
Consider the URL: https://example.com/en/page-a/
The Hint would trigger for this URL if it was missing the following hreflang annotation in the <head>:
AND if this self-reference was not present in the HTTP header, or in an XML Sitemap.
Why is this Hint marked 'Insight'?
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing.
It is not necessary for URLs to include a hreflang self-reference, but it could be considered helpful to include one.
If you did want to include self-referenced hreflang, you'd need to manually update the pages to add them in, or fix the script that generates the hreflang (depending on how the site is set up).