Directive or an assembly reference

Hi,

Why is not Visual Studio 2017 or Re-Sharper able to find a using directive for StackTrace as an example?

In my project VS 2017 I see Dependencies, SDK, Microsoft.NETCore.App; shouldn’t VS 2017 and Re-Shaper be able to find the using directive System.Diagnostics here? Or do I miss something here?

Thank you,