This error occurs when the tool is unable to restart the service. Typically, the service is stopped and restarted before and after completing the caching task to update or reset some parameters within the service configuration files and may have failed to restart the configuration.
Close the tool and restart the service manually. If you see errors while restarting the service, examine the server logs for errors. The log files are located at <ArcGIS Server Installation directory>\server\user\log on the SOM machine.