This error usually occurs because the website content contains both secure and insecure content.
For example the page code might include an image or code that pulls from a non-secure URL. To resolve this issue make sure that the page code does not pull data directly from a non-secure URL.
To identify non-secure elements view the source code of the page and search for the text src=”http://
Also check all css and js files are loaded from a https source.
This will then highlight elements on your page being loaded from a non-secure URL.
You may also want to consider using protocol relative links.