The LAW is an interpretive concept

Dworkin’s  point is that when judges disagree over the correct decision in a difficult case, they are disagreeing over the correct interpretation of the law – over what the law is. 

There is not in such cases a plain fact as to what the law is, which anyone with enough information can discover. But that does not mean that there is no law at all, so that judges can make it up. Law, as Dworkin puts it, is an interpretive concept