Hi Omkam, have you made sure there are no IP addresses referencing the VPN interface, under Policy Objects --> Addresses? Any addressing related to the interface will prevent you from removing it I believe. Also check there are no static routes still in place for that VPN, this caught me out a few times.
I see, well that should do it. What about deleting it via the cli. if you have tried that already them maybe this will help see the tunnel configurastion. get vpn ipsec stats tunnel or diagnose vpn tunnel list name $VPN_NAME. Hope you get it sorted.
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.