Additional Study Information: The purpose of this study is to evaluate the safety and efficacy of venetoclax and azacitidine compared to Best Supportive Care in patients with Acute Myeloid Leukemia (AML) after an allogeneic stem cell transplant (SCT). This is the first randomized study comparing venetoclax in combination with azacitidine versus best supportive care in subjects with AML who have just received an allogeneic stem cell transplant. Randomization is a process in which the computer makes a decision based upon chance that decides which study treatment you will be assigned. Best supportive care includes a few different medications that are typically given after receiving a stem cell transplant. Venetoclax in combination with azacitidine may produce the same results as best supportive care, it may work better than best supportive care, or may not work as well as best supportive care. Venetoclax has been approved by the United States Food and Drug Administration (FDA) to treat specific medical conditions but is currently not approved to treat AML after undergoing SCT. Therefore, the use of venetoclax is experimental for the purposes of this study. Azacitidine is FDA approved for the treatment of Myelodysplastic syndromes but is currently not approved to treat AML when combined with venetoclax.