What triggers this issue?
This issue reports HTTPS pages linking to HTTP URLs on your website.
Why is it important?
There is nothing particularly wrong with linking to an HTTP (non-secure) URL when it redirects to its HTTPS (secure) version. Although you can avoid this redirection by linking to the HTTPS page directly.
However, if an internal link on your website brings people to an HTTP URL, modern browsers will show a warning about a non-secure page. This can damage your overall website authority and user experience.
How to fix it?
Check the "Internal outlinks" column in this report for the URLs with http:// linked from the page.
You should then edit the links on the affected pages so that they point to HTTPS pages.
Even if every HTTP URL on your website redirects to its HTTPS version, linking to an HTTP URL will add an unnecessary redirection for users and crawlers. You should link to HTTPS URLs directly where possible.