CS0234: The name of the namespace or the type 'Helpers' does not exist in the namespace 'System.Web'. Need an assembly reference?

I am creating an ASP.NET MVC project with SharpDevelop and when compiling, it opens the list with the project folders in the browser. But when I click on one of my Views .cshtml or even the HOME folder, the following error appears:

Linha 23:     using System.Web.WebPages.Html;

I tried to delete it, but still the error persists.