Etavopivat is a new medicine under development for treating blood disorders like sickle cell disease and thalassaemia. Sickle cell disease and thalassaemia are inherited blood disorders that affect haemoglobin. Haemoglobin is the protein that carries oxygen through the body. This study is looking into how safe treatment with etavopivat is and how well it works over a long period of time. The study will last for up to 264 weeks, but it will end earlier if etavopivat is approved in the participant's country.