The purpose of this study is to find if adding the study drug, nivolumab (also known as OPDIVO), after standard chemotherapy {(mitomycin-C and 5-fluorouracil 5(-RU) or capecitabine) or 5-FU and cisplatin} and radition will prevent the anal cancer from returning. Nivolumab is a drug that may turn on the body's immune system to attack any cancer cells that may remain after chemotherapy and radiation. The addition of nivolumab may help prevent your cancer from returning, but it could also cause side effects. This research study will allow researchers to find out whether this different treatment is better, the same, or worse than the usual treatment for anal cancer.