The tool specified is not valid. Either the tool does not exist, or the ArcPy is not aware of the tool. In scripting, ArcPy only knows about the tools in toolboxes that it has been made aware of.
To confirm whether ArcPy is aware of a tool or toolbox, use the ListTools function or ListToolboxes function. To add a toolbox to the current geoprocessing session, use the ImportToolbox function.