“the King’s Valley.” This is most likely the Kidron Valley, on the east side of Jerusalem, or where the Kidron Valley and the Hinnom Valley meet (cp. Gen. 14:17).
“I have no son.” According to 2 Samuel 14:27, Absalom had three sons, so apparently by this later time in his life they had passed away. While tragic, that would not have been terribly unusual. Naomi lost both her sons in their teenage years after they were married but before they could have children (Ruth 1:3-5). In the ancient world children often died from various causes that they would not die from today. Not only were things such as appendicitis deadly, but many people died of injuries and infections that are easily treated today.
“Absalom’s monument.” The Hebrew is idiomatic: “Absalom’s hand.” The word “hand” refers to what the person had done and memorialized it. The word “hand” is commonly used for memorials or monuments.