Check the VPN Client Phase 1 algorithms to be consistent with the gateway (or peer) Phase 1 algorithms.
Check the IP address range.
Check the Local and remote IDs. (You can specify those if there are multiple tunnels between the two)
Note: the local ID on one site is the remote ID on the other site and conversely !
Check the PFS to be activated or not on both peers
Also verify the preshared key and try to avoid special characters.
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.