C# EF Required Attribute not recognized

I am creating an app that uses a database (SQLite). I am using entity framework and ADO.NET to interact with it. I have a seprate Models project in my app that contains all of my Database models.

Now I want to mark some of my class properties as required to reflect the "NOT NULL" option in my database. But if I add the [Required] Attribute from DataAnnotations namespace I get a compiler error saying it cannot be resolved. Here is how my class looks like :

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

namespace ReflowModels
{
public class Tag
{
    public Tag()
    {
        this.Options = new HashSet<Option>();
    }
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }
    public ICollection<Option> Options { get; set; }
}
}

I have also added a reference to EntityFramework.dll in my project.