More accurately, setting snapshot policy to on will let backup continue but log a warning. Setting it to required will abort the backup in case VSS does not work:
From the Duplicati command line help (v2.0.65): snapshot-policy : Controls the use of disk snapshots Type: Enumeration, Default value: off This settings controls the usage of snapshots, which allows Duplicati to backup files that are locked by other programs. If this is set to “off”, Duplicati will not attempt to create a disk snapshot. Setting this to “auto” makes Duplicati attempt to create a snapshot, and fail silently if that was not allowed or supported. A setting of “on” will also make Duplicati attempt to create a snapshot, but will produce a warning message in the log if it fails. Setting it to “required” will make Duplicati abort the backup if the snapshot creation fails. On windows this uses the Volume Shadow Copy Services (VSS) and requires administrative privileges. On linux this uses Logical Volume Management (LVM) and requires root privileges.
Personally I have it on “on” so it logs errors but if VSS fails (do not think I have seen it happening when proper permissions are given, yet) it will still backup whatever it can without it.