Microsoft has recently moved the Windows ADK download page to docs.microsoft.com, causing earlier versions of the ConfigMgr Prerequisites Tool to crash when attempting to load the available Windows ADK version in the Online mode. This issue has been addressed in the latest version.
Download ConfigMgr Prerequisites Tool 3.0.2
You can download the tool from the TechNet Gallery, following the URL below.
From this version and onward, ConfigMgr Prerequisites Tool source code will be available on GitHub:
Included in the download from the TechNet Gallery is a PDF with more details about each section of the tool.