Chrome - You connection to this site is not fully secure

I think you get that message when you use https, but Google Chrome cannot correctly verify your certificate chain. You might want to make sure you have followed the instructions on that end fully, and that all certificates are installed correctly.

If/when you have, this is how you can check that every request uses https:

• Open the website on Google Chrome
• Press F12
• Open the network tab
• Press f5 to reload everthing
• There it has all the information you should need. Hover over a request to check whether it uses http or https

In addition to Daniël's answer, I also found a insecure future form submit can cause this warning.

My web page has below form

<form action="http://example.com/..."  >
    <input >... 
</form>

Changing the action's value from http to https fixed the warning.

Or change to a "relative" protocol.

<form action="//example.com/..."  >

Also, in Chrome, hit F12 to open developer tools, go to Console, there is the details of the warning