This clinical trial is a Phase 2/3 study that will determine the recommended dose of mitapivat and evaluate the efficacy and safety of mitapivat in sickle cell disease by testing how well mitapivat works compared to placebo to increase the amount of hemoglobin in the blood and to reduce or prevent the occurrence of sickle cell pain crises. In addition, the long-term effect of mitapivat on efficacy and safety will be explored in an open-label extension portion.