Wisdom is the quality that Jonas will acquire during his training to be Receiver of Memory.
In Jonas’s community, every child is assigned a job at age twelve in a special ceremony known as the Ceremony of Twelve. Jonas is selected to be the new Receiver of Memory, which is a very important job of high honor in the community.
"The fourth essential attribute," the Chief Elder said, "is wisdom. Jonas has not yet acquired that. The acquisition of wisdom will come through his training. (ch 8, p. 63)
Wisdom is something usually acquired with experience and time, and sometimes age. Since Jonas has had the same experiences as everyone else, he has not acquired it yet. However, as he gets the memories from The Giver, he will definitely acquire wisdom. His training will involve him getting memories from back and back and back. All of the community’s memories from all time will make up his wisdom, as he slowly experiences them.
Although Jonas did not have wisdom, he was a reflective kid even before he was chosen. It did not take him long to learn from The Giver, and from his memories.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.