This site is not secure

 
Dear Concern,
I am facing this errror "This Site is not secure" when accessing, capture is attached. I have a fortiGate Firewall and a FortiWeb firewall and the server behind the fortiWeb. I want to configure fortiweb as a reverse proxy mode. when i had done the configuration like virtual ip, server pool, server policy so after when access the server via public ip so i received this error.
Let me explain you the connectivity.
I have an internet connection which is terminate on fortigate and I have another Public IP for VIP configuration so I configure the VIP to map Public to Private IP ( which is Virtual IP Address for accessing the server.). FortiWeb is behind the FortiGate firewall and fortiweb has configured a server policy which is virtual IP map on Actual server IP as you know Reverse Proxy mode. User's request from internet is reachable to the server the this page can't go forward. can you help me that how to create Certificate and resolve the issue.

 For your reference I am using this scenario as it is.