Your project is not referencing the ".NETPortable,Version=v4.5,Profile=Profile259" framework. Add a

i want to add nuget packages to my portable class library project ,

then i add a project.json to my portable class library project ,but vs 2017 build failed, error message is  following:

Your project is not referencing the ".NETPortable,Version=v4.5,Profile=Profile259" framework.

Add a reference to ".NETPortable,Version=v4.5,Profile=Profile259" in the "frameworks" section of your project.json,

and then re-run NuGet restore.

so ,i search from bing, i got a post:

then i add following json to project.json file:https://forums.xamarin.com/discussion/80045/error-using-nuget

".NETPortable,Version=v4.5,Profile=Profile259": {}

and keep only ".NETPortable,Version=v4.5,Profile=Profile259": {}" under "frameworks" node,like this:

{
  "dependencies": {
    "Portable.MvvmLightLibs": "5.0.2"
  },
  "frameworks": {
    ".NETPortable,Version=v4.5,Profile=Profile259": {}
  },
  "runtimes": {
    "win": {}
  }
}

then,i build my project again,it success!