This is a study to evaluate the safety and potential anti-tumor activity of an investigational agent called ARV-806 in Adults with Advanced Cancer having a specific KRAS mutation. This is an open-label study which means that participants and study staff will know that all participants will receive ARV-806. Researchers think that ARV-806 can work by breaking down a specific protein with a mutation that is present in some tumors, which might help prevent or slow tumors from growing. This will be the first time ARV-806 will be used in people. The investigational drug will be given through a vein. This is called intravenous (IV) infusion. This study will include 2 parts. In Part A (Phase 1), different small groups of participants will receive lower to higher doses of ARV-806. Adults with advanced cancers having a specific KRAS mutation will be included. In Part B (Phase 2), participants will be assigned to receive one of up to 2 dose levels decided by the information from Part A. Part B will include participants with advanced pancreatic ductal cancer having a specific KRAS mutation.