This isn’t a contradiction. In Matthew, Jesus says no sign will be given except the sign of Jonah, which refers to His future death and resurrection. In Mark, the exception is simply omitted and the statement is summarized as a refusal to give the Pharisees the sign they were demanding. Ancient biographies routinely condensed speeches without intending to record every word. Mark doesn’t say, “not even the sign of Jonah,” he just leaves the exception out. A summary that omits a detail is not a contradiction unless it explicitly denies that detail.