The client and server don't support a common SSL protocol version or cipher suite.
I saw https://forum.fortinet.com/tm.aspx?m=104586
Is FortiGate-60E Low-Encryption support https WebGUI?
This issue usually involves a problem with your web browser or your site’s SSL certificate. The browser’s telling you that because it’s trying to tell you there’s a problem with the certificate the website is using for HTTPS, so “this site can't provide a secure connection”. In all cases the end-to-end encryption is still going to work just because HTTPS can’t function without it. There is no definite guide for managing this error.
Two possible options to get rid of this issue
Use Self Sign certificate
Remove domain security policy:
Steps for Chrome :
Go to : chrome://net-internals/#hsts
Query HSTS/PKP domain for localhost
Use Delete domain security policies option to delete configuration for localhost
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.