Windows 7-10 x64, Notepad++ 5.9.5 Solution:
It has to do with the libTidy.dll not being included in the current distributions.
However, it was available in earlier versions.
Solution is to download the 5.9 zip, then
copy one of the following folders:
ansi\plugins\Config\tidy or Unicode\plugins\Config\tidy
to your current Notepad++\plugins\Config folder
Also, if you install to the c:\Program Files or c:\Program Files (x86) directories, you will need to adjust the permissions on the tidy folder to allow non-administrator access