Ensure that the UNC path being mapped to is correctly entered on the Drive Mapping element. Verify that the target machine where the share resides is online and that the folder is shared with the proper permissions. Ensure that the user has the proper permissions to map to the target share.
Copy the UNC path that is used on the Drive Mapping element and paste it into the Run box (go to Start | Run) to check if the UNC path can be resolved and opened. If the share cannot be accessed, from Start | Run, try using the FQDN or the IP Address of the server. If the share can be accessed by either the FQDN or the IP Address but not the NetBIOS name, flush the DNS resolver cache (open a CMD prompt and type IPCONFIG /FLUSHDNS) on the client machine and reboot it.