**Study Summary** This clinical trial is for children and teens aged 6 to less than 18 who have *functional constipation* (FC), which means they don't have regular bowel movements and experience discomfort. The study tests if a medicine called plecanatide can help. Plecanatide is taken as a pill once a day and works by increasing fluid in the intestines to make stools softer and easier to pass. The trial compares different doses of plecanatide with a placebo (a pill without medicine) to check if plecanatide helps more. Participants will: - Take the medicine or placebo every day for 8 weeks and keep a daily diary about their bowel movements. - Visit the clinic for check-ups and tests. - Give blood samples to help researchers understand how the medicine works in the body. **Key Points:** - The study lasts about 14 weeks total, including a 2-week follow-up period. - Participants must attend several clinic visits for safety checks. - They will share their experiences and may receive a placebo instead of the active drug. This study aims to find a new treatment for young people with constipation, who currently have limited options.