In some instances, the ApplicationPlugins folder for Autodesk products is not created. When the ArcGIS for AutoCAD installer performs its checks it might present the user with the following notification:
ArcGIS for AutoCAD setup requires Autodesk AutoCAD. Please refer to the installation notes for more information.
The folder, C:\Program Files\Autodesk\ApplicationPlugins, is not created when the Autodesk product is installed.
Create the missing folder and rerun the ArcGIS for AutoCAD installation.