Why can't I reference System.ComponentModel.DataAnnotations?

You have to reference the assembly in which this namespace is defined (it is not referenced by default in the visual studio templates). Open your reference manager and add a reference to the System.ComponentModel.DataAnnotations assembly (Solution explorer -> Add reference -> Select .Net tab -> select System.ComponentModel.DataAnnotations from the list)

If using .NET Core or .NET Standard

use:

Manage NuGet Packages..

instead of:

Add Reference...

To Reference System.ComponentModel.DataAnnotations

In a code file to have Using System.ComponentModel.DataAnnotations; at the top of the file such as:

using System.ComponentModel.DataAnnotations;

Add a .NET reference to your project by right clicking the project in solution explorer:

Hope this helps! This question helped me.

If you don't have it in references (like I did not) you can also add the NuGet System.ComponentModel.Annotations to get the assemblies and resolve the errors. (Adding it here as this answer still top of Google for the error)