Unity is a very strange tool from .NET user's view. It's for Unity doesn't provide the NuGet package manager as a first-party tool. Unity users should work with a third-party NuGet package manager. So, if you need it setup to your unity project.
The procedure for installing NuGet Package Manager to Unity is as follows.
Download binary in release page
Drag and drop to your Unity project
Restart your Unity project
If you like command line, try this example shell script. It's without handling error.
Result is here.
Finally, i hope that the NuGet package manager will be standard equipment on Unity.