This trial is studying tucatinib plus KADCYLAź (T-DM1) to find out what the side effects are and if it works for treating HER2 positive breast cancer. A side effect is anything the drug does to your body besides treating your disease. We do not yet know if tucatinib plus T-DM1 is better or worse than other treatments. Learning about the side effects of tucatinib plus T-DM1 and how it works will help us understand if tucatinib plus T-DM1 is better or worse than other treatments. To do this, we will compare 2 groups in this study. One group will get tucatinib plus T-DM1 and the other group will get a placebo plus T-DM1. A placebo is a pill that doesn?t have any drug or medicine in it. We will choose your group randomly, like tossing a coin.