"This Site Is Not Secure Message"

We are a new club (07458672 - Sarasota Bay Simply Speaking). A week or so ago I set up the basics of our FTH site. I changed the site URL to be:

www.sarasotabaysimplyspeaking.toastmastersclubs.org

I and a number of members keep getting a message when we attempt to log in which reads

"This Site is Not Secure
This might mean someone's trying to fool you or steal any info you send to the server. You should close this site immediately."

​I have clicked past this and got into the page (and now I am worried) but other members are not as willing to accept risk...could you shed any light on this issue and what it would take to correct it?

Thanks

We do not support www in your club website address.

Your website address is sarasotabaysimplyspeaking.toastmastersclubs.org