And I directed my attention to discern what I now heard, that free will was the cause of our doing evil, and Your righteous judgment of our suffering it. But I was unable clearly to discern it. So, then, trying to draw the eye of my mind from that pit, I was plunged again therein, and trying often, was as often plunged back again. But this raised me towards Your light, that I knew as well that I had a will as that I had life: when, therefore, I was willing or unwilling to do anything, I was most certain that it was none but myself that was willing and unwilling; and immediately I perceived that there was the cause of my sin. [Confessions, VII.3]
If we do not have free will, it is not possible for us to sin—because it is the cause of sin (as St. Augustine says here). That is not to say that we are compelled to sin by virtue of the fact that we have free will (which would contradict the very idea of free will), but rather that if we did not have free will, we could not sin.