Project ‘ClassLibrary1.csproj’ targets ‘netcoreapp2.1’. It cannot be referenced by a project that targets ‘.NETFramework,Version=v4.7.2’

You are not able to reference:

• .NET Core libraries from .NET Framework
• .NET Framework libraries from .NET Core

If you want to share code between those two libraries you have to use .NET Standard libraries.

.NET Standard is a kind of interface, a versioned list of APIs that you can call, .NET Framework and .NET Core implements this standard

Please see for reference:
.NET Standard