.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.