Are you running version 7.2.x? Starting from 7.2.0 AV and IPS packages are now signed by the Fortinet CA to ensure authenticity of the packages. FortiOS is expected to verify the signature before installing the update, may be for some reason this particular update file had issues with signature and resulted in the error (could be on Fortiguard,Fortigate or the connectivity).
Are you continuously getting this error or it was a one time occurance. Usually the updates happens multiple times a day and if the issue might have fixed with next update.
If the issue is fixed next update and errors are not repeating we may ignore these alerts.
You can also use "diagnose autoupdate versions" to verify the databases are updated.
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.