Mix content is the case when an HTML page is loaded via a secure connection, but some embedded resources, like images, scripts and styles are loaded from non-secure origins, browsers show "unsecure site" error message, scaring your users away. So, we must locate the source to eliminate the isse. The issue also arises if you have too old websites, and images uploaded in before you moved your site to HTTPs are still hosted on HTTP.
Now to remove the error and track source, you can use the following methods.
Use Mozilla Firefox developer tool
Just open your website in Firefox and right click and open the inspect and select console. Here you can see all content that is passed over HTTP as errors.
Use Online tools
You can also scan your website through online scanning tools.
Using Advanced CLI tools
You can also use advanced CLI tools to scan through your website recursively and see for the HTTP content.
How to fix your Site?
The best strategy to avoid mixed content blocking is to serve all the content as HTTPS instead of HTTP. For your own domain, serve all content as HTTPS and fix your links. Often, the HTTPS version of the content already exists and this just requires adding an "s" to links - http:// to https://. Otherwise just upload the media files again.