After installing the 64-bit Background Geoprocessing Upgrade, and running a Python toolbox in the background, the following error is shown in the results window and the tool fails to run:
"Error 000819: The tool is not assigned."
A third-party Python module that is referenced in the toolbox is not installed in the 64-bit Python folder for access by background geoprocessing. When the Python toolbox tool is run, 64-bit background geoprocessing cannot access the module in the 32-bit Python folder.
The toolbox appears to be valid with correct syntax because ArcGIS has access to the 32-bit modules, but does not check for the 64-bit modules.
Edit the Python toolbox's tools so they always run in the foreground by assigning the "self.canRunInBackground" line of code with False. For example:
"""Define the tool (tool name is the name of the class)."""
self.label = "Tool"
self.description = ""
self.canRunInBackground = False