Here's a more complete answer from a draft of a knowledgebase article about to be published:
Title
Problem: "Error 1606: Could not access the network location" encountered when attempting to add the installation of ArcGIS for AutoCAD 400Edit Title
Description
When installing version 400 of ArcGIS for AutoCAD, you encounter the error.
Cause
ArcGIS for AutoCAD is installed into the %ProgramFiles%/Autodesk/ApplicationPlugins folder. If this folder is missing or not accessible by the current user, ArcGIS for AutoCAD cannot be installed
Solution
There are two workarounds:
1. Recommended: Reinstall AutoCAD to recreate the folder. OR,
2. Create a new folder titled “ApplicationPlugins” in the %ProgramFiles%/Autodesk (this may have unintended consequences when installing and uninstalling AutoCAD plugins)