About the issue
If your website contains any elements that are not secured with HTTPS, this may lead to security issues. Moreover, browsers will warn users about loading unsecure content, and this may negatively affect user experience and reduce their confidence in your website.
How to fix it
Only embed HTTPS content on HTTPS pages.
Replace all HTTP links with the new HTTPS versions. If there are any external links leading to a page that has no HTTPS version, remove those links.