This occurs if some links on the page are using unsecured HTTP protocol. You need to change each and every link on the destination page as HTTPS.
To find unsecured links on your website, refer to this link enter the domain at the search bar.
Please follow the steps below to avoid this message.
1. Change all HTTP:// resource links to https://. The doctype is not a problem. All others (images, CSS, javascript, iframe pages) should be https://.
2. Change all about: blank links to a blank file that exists and has an https:// link.
3. Check if any iframes have # for src or if src is left blank. Change them to something that is an https:// link.
