When an error occurs while attempting to publish from Revit, with an error message of "Revit cannot run the external application Assemble," the error can be resolved by updating the Microsoft.net framework.
To update the Microsoft.net framework
1. Uninstall the Assemble add-in from your installed Programs and Features, accessible through the Control Panel.
2. Install the Microsoft.net update.
3. Re-install the Assemble add-in.
4. Launch Revit and access the Publish tool from Assemble.