The correct answer and the way that the symbol of the desert affects the meaning in this passage of Paulo Coelho's "The Alchemist" (1988) is The desert represents the struggles everyone must face if they want to realize their dreams.
The image of a desert brings to our mind the idea of harshness, of hardness, of an arid landscape and of the challenge of crossing a wasteland with the dangers that it entails if we are not prepared to do it.
In this context, the idea of the desert and the struggles that you must face to cross it, and in this excerpt achieve your dreams, is what prevents the crystal merchant to try and make his dreams real. If you want to fulfill your dreams, you must struggle and face the challenges of the symbolic desert that stand in your way.