For some reason, the nuget package manager is not always installed by default. I came across this problem a few times already.
What you want to do is this.
First: You need to unistall npm(nuget package manager), you did this already, you can try this again, just to be sure :)
Visual studio -> Tools -> Extensions and Updates -> Npm -> unistall
Second: after you unistall npm, you need to install it again to use it (ofc). There are several ways to install npm:
1) install it under Visual studio -> tools -> extenstions and
Updates> npm -> install
2) GOTO http://nuget.org, search the install nuget link and click
on it, downlaod the .vsix file and install in on your visual studio
Third: after the installation, close and re-open visual studio, hopefully you will never get that error again.