Why can not I access the methods of a class library?

The practice program that I am doing consists of creating an English deck and writing the cards in console (The King of Hearts -for example-), but they do not come out in order, but at random. I have a class library with 2 classes and 2 enum to do this project. But when compiling the code of the main program (image 1) the error marks me. I'm just starting on this, but I can not understand the reason for this error.

Here is the code of my class library .

class Deck
    {
        private Carta[] cartas;
        public Deck()
        {
            cartas = new Carta[52];
            for (int vPalo = 0; vPalo < 4; vPalo++)
            {
                for (int vRango = 0; vRango < 14; vRango++)
                {
                    cartas[vPalo * 13 + vRango - 1] = new Carta((Palo)vPalo, (Rango)vRango);
                }
            }
        }
        public Carta ObtenerCarta(int numeroCarta)
        {
            if (numeroCarta >=0 && numeroCarta <= 52)
            {
                return cartas[numeroCarta];
            }
            else
            {
                throw (new System.ArgumentOutOfRangeException("numeroCarta", numeroCarta, "El numero de cartas debe ser de 52!"));
            }
        }
        public void Shuffle()
        {
            Carta[] nuevoDeck = new Carta[52];
            bool[] asignado = new bool[52];
            Random source = new Random();
            for (int i = 0; i < 52; i++)
            {
                int destCarta = 0;
                bool cartaEncontrada = false;
                while (cartaEncontrada == false)
                {
                    destCarta = source.Next(52);
                    if (asignado[destCarta] == false)
                    {
                        cartaEncontrada = true;
                    }
                }
                asignado[destCarta] = true;
                nuevoDeck[destCarta] = cartas[i];
            }
            nuevoDeck.CopyTo(cartas, 0);
        }
    }
class Carta
    {
        public readonly Palo palo;
        public readonly Rango rango;
        public Carta(Palo unPalo, Rango unRango)
        {
            palo = unPalo;
            rango = unRango;
        }
        private Carta()
        {

        }
        public override string ToString()
        {
            return "El/La " + rango + " de " + palo;
        }
    }