This main aim of this trial will be to study the blood levels (known as pharmacokinetics) of the study drug tazemetostat. The pharmacokinetics of the study drug in participants with advanced solid tumors and moderate or severe hepatic (liver) impairment will be compared with participants with advanced malignancies and normal hepatic function. An advanced malignancy is a cancer that is unlikely to be cured or controlled with treatment. The trial will also study the safety and the level that adverse effects of the study drug can be tolerated (known as tolerability).