The goal of this study is to learn if tilpisertib fosmecarbil (formerly known as GS-5290) is effective and safe in treating participants with moderate to severe ulcerative colitis. The study will compare participants in different treatment groups treated with tilpisertib fosmecarbil with participants treated with placebo. The primary objective of this study is to demonstrate the efficacy of tilpisertib fosmecarbil, compared to placebo control, in achieving Clinical Response at Week 12.