This study is conducted to confirm whether etavopivat works well at reducing the number of Vaso-occlusive crisis VOCs (sickle cell pain crises) caused by obstructions in blood vessels in adults and adolescents living with sickle cell disease. The study will also evaluate how well etavopivat can reduce the damage to different organs, improve your exercise tolerance and reduce fatigue in people with sickle cell disease.The participants will either get etavopivat or placebo. Which treatment the participants will get is decided by chance. Etavopivat is a new medicine and is currently being tested in other studies in addition to this one. The study will last for about 2 years.