To assess the efficacy of ABBV-400 in each solid tumor indication, including advanced or metastatic hepatocellular (HCC), biliary tract cancer (BTC), pancreatic ductal adenocarcinoma (PDAC), esophageal squamous cell carcinoma (ESCC), triple negative (TN) breast cancer (BC), hormone receptor-positive (HR+)/human epidermal growth factor receptor 2 negative (HER2-) BC, and head and neck squamous cell carcinoma (HNSCC). The purpose of this study is to evaluate ABBV-400 in subjects with select solid tumors. This study will consist of multiple cohorts with each cohort investigating ABBV-400 at 3 mg/kg Q3W.