Yesterday I installed the Visual Studio 2015 Update 3 and began playing with a new ASP.NET Core API project. But the NuGet packages needed to be updated and when I tried to update them, they could not be found.
Clue: Pay attention to the little black flag in the upper right corner of Visual Studio 2015 () and install the .NET Core 1.0.0. VS 2015 Tooling Preview 2.
The install took a fair amount of time, but when it was done, I created a new ASP.NET Core API project and bada-bing, everything seems happy.