ER Wildtype

The goal of this research study is to compare a combination of two drugs, capecitabine and elacestrant to capecitabine alone as a treatment for advanced estrogen receptor-positive (ER+) breast cancer. This study is designed for participants with cancer that has previously stopped responding to medication in the class of therapy called CDK 4/6 inhibitors, including palbociclib, ribociclib, or abemaciclb. The names of the study drugs involved in this study are: * Elacestrant (a type of selective estrogen receptor degrader) * Capecitabine (a type of fluoropyrimidine antimetabolite)