In short
Jesus is judging not only the scribes and Pharisees standing before him, but the whole generation of Jews of his day, as corrupt and unfaithful. In turning away from the way of the living God, they fail to recognize the signs that Jesus performs (see also Matthew 12:24, Matthew 12:28).
Jesus is not speaking only to the few scribes and Pharisees standing before him. With the words of judgment spoken in Matthew 12:39 he addresses the entire generation of Jews of his day, exposing them as corrupt and unfaithful. They have turned aside from the way of the living God, and for that reason they fail to recognize the signs that Jesus performs (see also Matthew 12:24, Matthew 12:28).1
39 But he answered them, “An evil and adulterous generation seeks for a sign, but no sign will be given to it except the sign of the prophet Jonah.