"I made a series of wrong decisions about moderately recent books, and I've sold the rights to studios for ridiculous amounts of money and the films have never been made. That's the saddest thing of all, because they're locked up and no one else can make them"

John Le Carre is expressing regret over a series of choices he made regarding the sale of the rights to his books. He acknowledges that he sold the rights for a large amount of money, but that the movies were never ever made. This provides unhappiness for him, as he thinks that the stories he wrote are now locked away and no one else can make them. He is likely reflecting on the fact that he could have made more cash if he had offered the rights to various studios, or if he had actually waited to offer the rights until the films were really made. He is also most likely assessing the truth that his stories will never ever be seen on the cinema, and that he will never get to experience the happiness of seeing his work come to life.