The purpose of this study is to evaluate investigational treatments (study drug) for unresectable metastatic colorectal cancer to determine if any of these study treatments improve overall survival as compared to standard treatments. The goal is to determine the optimal dose level, safety, and tolerability for the study drug ABBV-400 in combination with fluorouracil, folinic acid, and bevacizumab. The study consists of two stages, Stage 1 - Safety Lead-In Dose Escalation and Stage 2- Dose Optimization. ABBV-400 is not FDA approved. Bevacizumab is FDA approved by the U.S. Food and Drug Administration (FDA) to treat various cancers but may not be approved for your type of cancer. Treatment for this study may be up to 3 years. The procedures include blood and urine samples, questionnaires, infusions, and CT scans. Risks include diarrhea, nausea, vomiting, fatigue, and numbness, tingling, or weakness in arms and legs. You may or may not receive a direct benefit from participating in this trial, however, information learned from the trial may help other people in the future.