Creating a Classifier Extension ?

I am trying to create a classifier extension, which is shown in the web link:

https://docs.microsoft.com/en-us/visualstudio/extensibility/creating-an-extension-with-an-editor-item-template 

The solution is built successfully without problems. but when I run it and try to open a new text file, the editing area of the new text file does not appear. 

if I try to open a new C# class file, then it gives me the following error message: 

Expexted 1 export(s) with contract name "Microsoft.VisualStudio.Text.Editor.ITextEditorFactoryService" but found 0 after applying applicable constraints. 

and sometimes the following meesage is shown: 

Roslynpackage did not load correctly.