The type Expression<> is defined in an assembly that is not referenced. You must add a reference to assembly ‘System.Core, Version=184.108.40.206, Culture=neutral, PublicKeyToken=
If these errors occur on an Mvc page for no reason (worked fine before), recompile your solution and look for an error in the (often now long) compile error list. You will probably find a piece of code missing brackets, parameters or such. Small syntax errors can twist Visual Studio (or rather the compiler) to mislead you to an error that has nothing to do with the real error (like in this case, referencing System.Core).