The Volume Shadow Copy Service (VSS) is a set of COM APIs that implements a framework to allow volume backups to be performed while applications on a system continue to write to the volumes.

If this service is not running on your machine, Zoolz will fail to back up your opened and locked files. To check this service status:

  1. Go to the Start menu on your machine.
  2. In the Search\Run filed type: services.msc and hit enter.
  3. Look for the Volume Shadow Copy Service, right click on it and select Properties. From the General tab, make sure that the Service startup type is set to Automatic and then start the service.
  4. Repeat the step 3 for the service Microsoft Software Shadow Copy Provider.

And also check the Windows Event Viewer for any recent errors related to this service. Here is how you can do that:
From Start menu → Control PanelAdministrative Tools → Double-click Event Viewer→ From the left panel of the Event Viewer, select Windows Logs then Application → Use the event ID and the error message to search for a suitable fix for it.