When scaffolding a controller in ASP Core 2.0 VS 2017 I get the following:
Could not get the reflection type for DbContext :
BE.Data.ApplicationDb
I had this issue too, when I was using the new C#7 feature (expression bodied members and Throw expressions) but by replacing my code with it's equivalent in C#6 it didn't give me that error anymore. Try to test your project without any C#7 new feature and probably it should works fine. I'm not sure what is the reason but I think The Roslyn packages that scaffolding depends on does not support C#7 new features yet.
As a workaround to let scaffolding works with C#7 I think you can by doing the following:
Microsoft.CodeAnalysis.CSharp.Workspaces
version 2.0.0 or higher