Permission denied (-455) means that the login that you used is not having legitimate permission to get connected to the SSLVPN. You will want to check the error in FortiGate with the following command when you connect to the VPN:
diag deb app sslvpn -1
diag deb app fnbamd -1
diag deb en
Ensure that the user belongs to a user group that has been configured with policy to allow the connection to your internal network.
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.