nuget configuration is invalid.

---------------------------
Microsoft Visual Studio
---------------------------
"Project" nuget configuration is invalid.

---------------------------
OK  
---------------------------

When you try to add the reference, via Manage Nuget Packages. You may get this above error message.

The solution is simple. Close the Visual Studio and open it again. This works most of the time. 

The License Agreement file, "C:\WINDOWS\system32\FPdotLicense.dat" does not exist.

---------------------------
License Agreement Not Found
---------------------------
The License Agreement file, "C:\WINDOWS\system32\FPdotLicense.dat" does not exist. Please install this file to continue.
---------------------------
OK  
---------------------------

Cause:
When you build the old solution in New Visual Studio 2015, you may get the above error message.


Solution:
The following folder path needs write permission for C:\WINDOWS\system32\ while building the solution. Its a license scheme for windows.