Possibly save 1 hour of your time: When you try to install a nuget package with a dependency on .NET Standard 2.0, it gives you this error.
.NET Standard 2.0 is supported by .NET Core 2.
You can right click your project >> select properties. Check to see what .NET Core you are using.
If you don’t have .NET Core 2.0, the easiest way is to download latest is to go into Visual Studio > Tools > Extensions and Updates > Under Updates > Product Updates > you should be able to see Visual Studio Update.
You can also download SDK and Runtime here https://www.microsoft.com/net/download/visual-studio-sdks
For more information on .NET Standard 2.0, please see https://blogs.msdn.microsoft.com/dotnet/2017/08/14/announcing-net-standard-2-0/