A stop sign can only tell you that you have to stop but it cannot stop the car unless you happen to hit into it as I did in a snow storm once. You need a set of working brakes to stop. Once you get the breaks you now have the power to stop. But the sign in and of itself had no power to make you stop. That is what law does.
Grace gives that power.
The whole of verses 2-14 is to answer that objection. Here Paul uses a different tense when speaking of sin. After having shown that a Christian, because of his union with Christ in His death and resurrection, cannot live a life of sin, the question comes up as to whether or not we can excuse acts of sin. The logic is that since we have the power now being under grace and not being able to fall under sins continuous dominion we can take acts of sin lightly. That is what he deals with in the next few verses.