The 'clr-namespace' URI refers to a namespace

Question

I'm using Visual Studio 2015 and added two folders in the root directory.

1. Model
2. ViewModel

and two classes:

1. Printer (in Model)
2. PrinterViewModel (in ViewModel)

I have already rebuild the project. The strange is that the Model.Printer namespace will be found, but not ViewModel.PrinterViewModel.

I read already this article The 'clr-namespace' URI refers to a namespace that is not included in the assembly