Check if your FortiOS Version is supported by Fortimanager.
Check out the FortiManager Support for FortiOS compatibility chart in the Fortinet Documentation Library. You may find the Compatibility with FortiOS chart in the Release Information.
In my case the FortiGate was running 7.0.9 and the FortiManager was running 7.0.4 and this is unsupported because i’ve tried to install the FortiOS Version 7.0.10
