This is a Randomized, Double-blind, Placebo-controlled, Multicenter, Parallel-group Study of the Efficacy and Safety of 2 Weight-based Treatment Groups of Plecanatide Versus Placebo in Children and Adolescent Participants 6 to Less Than 18 Years of Age With FC

The goal of this clinical trial is to learn if plecanatide can treat functional constipation in children and adolescents aged 6 to less than 18 years. It will also learn about the safety and pharmacokinetics of plecanatide in this population. The main questions it aims to answer are: * Does plecanatide increase the number of spontaneous bowel movements compared to placebo after 8 weeks of treatment? * What medical problems do participants experience when taking plecanatide? Researchers will compare low-dose plecanatide and high-dose plecanatide to placebo to see if plecanatide improves bowel movement frequency, stool consistency, and constipation-related symptoms. Participants will: * Complete a screening period with daily electronic diary entries to record bowel movements, symptoms, and rescue medication use * Take plecanatide or placebo by mouth once daily for 8 weeks * Continue daily electronic diary entries throughout the study * Attend clinic visits for physical exams, laboratory tests, and assessments of symptoms and safety * Provide blood samples for pharmacokinetic and safety evaluations * Complete questionnaires about constipation symptoms and quality of life

Inclusion Criteria:• Male or female children and adolescents aged 6 to \