When trying to install a package from our newly created private Nuget Feed i get the following error.
Could not install package \'GC.Timecode 1.0.0.3\'. You are tryi
Another possibility which I just worked through was that local Nuget tools can be too old.
I was trying to install https://github.com/alastairtree/LazyCache which is a .Net standard 2.0 project. My local project was .NET framework 4.7.2, which is compatible. And yet I got the error cited in the question.
The problem was that my VS2015 installation of Nuget was not the latest. Although under Tools > Extensions it did NOT show any available updates, there was in fact a much newer Nuget available at https://www.nuget.org/downloads (3.6x vs 3.4x I believe).
I installed that and the problem was resolved. Apparently the old Nuget did not understand what .NET Standard was.