Jonas is disturbed by the rule that he can lie, because no one in the community is supposed to lie and he begins to wonder if some of them can after all.
When Jonas receives his one page of instructions during the Ceremony of Twelve, he is “stunned.” He learns that he will not have free time, and he is exempted from rules governing rudeness. Yet while the exemption from rudeness “startled him” and the restriction from medication “unnerved him,” it was really the rule about lying that disturbed him.
His mind reeled. Now, empowered to ask questions of utmost rudeness—and promised answers—he could, conceivably (though it was almost unimaginable), ask someone, some adult, his father perhaps: "Do you lie?" (ch 9)
Jonas is concerned because the permission to lie shakes the foundation of his beliefs about his community. He realizes that there is no way he can really know if anyone who answers his question about lying is actually telling the truth.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.