I had the problem, that no Nuget packages could be loaded in a project on a computer with a freshly installed Visual Studio.
The problem was, that there were no package sources defined in Visual Studio. I would have expected, that the default Nuget store should be available after a fresh install, but it was not.
In the options, this can be easily fixed.
The source URL for the standard nuget.org is:
Once this was set up, I was able to run the project, nuget packages where downloaded and restored.