Additional Study Information: The purpose of the study is to see whether tomivosertib in combination with pembrolizumab treatment slows down the growth of cancers. The study drug, tomivosertib, works by inhibiting the activity of proteins in a pathway that allows tumor cells to grow and go unnoticed by your immune system. This study will evaluate the safety and antitumor activity of tomivosertib versus placebo in subjects with Non-Small Cell Lung Cancer. There are two study arms for this study. Treatment arm A, in addition to taking study drug you will continue to receive pembrolizumab as either 200 mg by infusion at every 3 weeks or 400 mg by infusion every 6 weeks, and treatment arm B, in addition to taking study drug you will begin pembrolizumab as either 200 mg by infusion every 3 weeks or 400 mg by infusion every 6 weeks. This is a double-blind study, which means neither you nor the study doctor will know if you are receiving tomivosertib or placebo.