public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public string Username { get; set; }
    public string Password { get; set; }
    public DateTime Created_at { get; set; }

    public  ICollection<Device> Devices { get; set; }
}

public class Device
{
    public int Id { get; set; }
    public string Name  { get; set; }
    public string Type { get; set; }
    public string Gateway { get; set; }
    public DateTime Created_at { get; set; }

    public int User_id { get; set; }
    public User User;

}

HasMany(t => t.Devices)
            .WithRequired(t => t.User)
            .WillCascadeOnDelete(true);

The expression 't => t.User' is not a valid property expression. The expression should represent a property: C#: 't => t.MyProperty'  VB.Net: 'Function(t) t.MyProperty'.