Symptoms

Parallels Desktop displays the following error on attempting to revert to the snapsot when the Virtual machine is running:
 

Unable to revert to this snapshot

Resolution

1. Shut down your Virtual Machine and then try to revert to snapshot.
2. If your Virtual Machine is already shut down, make sure its virtual hard disk is not mounted to Finder in Mac OS X:

• Open Finder
• Check if there are any Virtual Hard disks mounted    

• Unmount virtual hard disk(s) from Finder and try to revert to a Snapshot again