The drug being tested in this study is called Pevonedistat. Pevonedistat is being tested to treat people with myelodysplastic syndromes MDS (including nonproliferative chronic myelomonocytic leukemia [CMML]) and AML (acute myeloid leukaemia) as a single-agent and in combination treatment with azacitidine. This study will look at the safety and tolerability, the recommended phase 2/phase 3 dose of pevonedistat administered in combination with azacitidine, pharmacokinetics and response to treatment in participants who take single agent pevonedistat compared to participants who take pevonedistat and azacitidine.