LINQ to Entities does not recognize the method 'System.Object get_Item(System.String)' method, and this method cannot be translated into a store expression.

The Linq query is ultimately transformed into an SQL query and LINQ doesn't know what to do with Session["UserName"] (that gets the "UserName" item).

A common way to workaround this is just to use a local variable to which you'll assign Session["UserName"] and that you'll use in your Linq query...

like  

string loggedUserName = Session["LogedUsername"].ToString();
  var userdetail =
dc.faculties.Where(a => a.F_UserName.Equals(loggedUserName)).FirstOrDefault();