Does not contain a definition for "Add"

I'm trying to create something similar to that in the this thread, but I'm running into an error:

'System.Collections.Generic.IEnumerable' does not contain a definition for 'Add' and no extension method 'Add' accepting a first argument of type 'System.Collections.Generic.IEnumerable' could be found (are you missing a using directive or an assembly reference?)

This is my code:

[HttpPost]
public ActionResult Create(ANIME anime)
{
    var db = new MainDatabaseEntities();
    var newanime = new ANIME
    {
        ID_AN = anime.ID_AN,
        TITLE_OR = anime.TITLE_OR,
        TITLE_EN = anime.TITLE_EN,
        GENRES = new List<GENRES>()
    };

    foreach (var selectedAnime in anime.GENRES.Where(c => c.isSelected))
    {
        var genre = new GENRES { ID_GE = selectedAnime.ID_GE };
        db.GENRES.Attach(genre);
        newanime.GENRES.Add(genre); <--- this is the error line
    }

    db.ANIME.Add(newanime);
    db.SaveChanges();
    return RedirectToAction("Index");
}

ANIME:

public partial class ANIME
{
    public int ID_AN { get; set; }
    public string TITLE_OR { get; set; }
    public string TITLE_EN { get; set; }

    public virtual IEnumerable<GENRES> GENRES { get; set; }
}

GENRES:

public partial class GENRES
{
    public int ID_GE { get; set; }
    public string GENRE { get; set; }
    public bool isSelected { get; set; }
    public virtual ICollection<ANIME> ANIME { get; set; }
}

The line contains the error.newanime.GENRES.Add(genre) in HttpPost I includedusing System.Linq to every model and controller, but it makes no difference. Any suggestions on how to fix this?

EDIT:

A new fault appeared after this was fixed. I believe it has nothing to do with the previous one, but I don't want to spam threads.

Message of error:

The entity or complex type 'MainDatabaseModel.GENRES' cannot be constructed in a LINQ to Entities query.

Associated code:

public ActionResult Create()
{
    var db = new MainDatabaseEntities();
    var viewModel = new ANIME
    {
        GENRES = db.GENRES.Select(c => new GENRES
        {
            ID_GE = c.ID_GE,
            GENRE = c.GENRE,
            isSelected = false
        }).ToList()
    };
    return View(viewModel);       
}