You cannot add your cream cheese to your instant pot until the end of the cooking phase. Once everything has cooked and is no longer boiling, you can add the cream cheese, chopped into small chunks, and stir it until it dissolves. Like any dairy product, your cream cheese will curdle if you put it in initially and let it boil with the rest of the ingredients. Also, make sure it is at room temperature. The cheddar cheese should also be added at the end for the same reason.