This is an open-label, Phase I, dose-escalation study to determine the maximum tolerated dose (MTD) and the recommended phase two dose (RPTD), and to assess the safety, preliminary efficacy, and pharmacokinetic (PK) profile of ABBV-176 for participants with advanced solid tumors likely to express Prolactin Receptor (PRLR). The study will consist of 2 cohorts: Dose Escalation and Expanded Recommended Phase 2 Dose.