Related document.
https://docs.fortinet.com/document/fortigate/7.0.0/new-features/538358/use-ssl-vpn-interfaces-in-zon...
Solution
Go to Network -> Interfaces -> Create New -> Zone.
Select 'ssl.root' in zone. Make sure 'ssl.root' is not using in any firewall policy. If not, it will not be possible to see 'ssl.root' appear in the list.