At https://sourceforge.net/p/notepad-plus/discussion/482781/thread/2eeba3f3/ I found a solution. I have downloaded the zip file https://github.com/downloads/davegb3/NppTidy2/Tidy2_0.2.zip. Expanding this zip folder has Tidy2.dll. I copied this Tidy2.dll file to C:\Program Files (x86)\Notepad++\plugins. Then I verified that this plug in is showing up in the nodepad++ menu.
But I am interested to know 1. why it fails to install as a plugin from notepad++ menu? 2. Is there a solution than the above hack I mentioned?