How Do You Reference a .NET Standard Library from a .NET Framework 4.5 Console Application in Visual Studio 2017?

.NET Framework 4.5 only supports using .net standard libraries targeting .NET Standard 1.0 or 1.1. Since your library targets 1.6, the tooling does the right thing here and errors out (since your library may use APIs not available in .NET Framework 4.5). If you published the library as NuGet package and consumed it via a package reference, the package restore would error out as well (with an error saying that the package is incompatible).

There is some confusion about which .NET Standard version a .NET Framework version supports especially since there is preview tooling available ("2.0") that changes these versions. The ".NET platforms support" table in the documentation therefore contains two lines about the supported versions. In your case however, both versions limit .NET Framework 4.5 to .NET Standard 1.1.

for .net framework projects to be compatible with .net standard libraries you must acquire the NETStandard.Library from the nuget.
Now i cannot find any official resource that states exactly why this is a must, but from what i understand the NETStandard.Library has the necessary links to make a map from .NET Standard API's to .NET Framework.
If you want more info i suggest to read the official docs of NET Standard.