Additional Study Information: This study is being done to answer the following question: How does the combination of copanlisib and eribulin compare to eribulin alone to lengthen the time you can live with your metastatic triple-negative breast cancer without your cancer getting worse? We are doing this study because we want to find out if this approach is better or worse than the usual approach for your triple-negative breast cancer. The usual approach is defined as care most people get for metastatic TNBC.