Removing knockouts in composite granite sink
As brutal as it seems, the prescribed method by FRANKE is to use a blunt ended large punch and a hammer.  Set the punch dead center in the cutout and strike it soundly. Don't be too tentative, give it a good shot! Place the punch on the top. If you have a carbide hole saw, pilot a small hole dead center from the side already drilled then finish it off from the top, using the pilot hole.

When I first saw my plumber do this on a kitchen job, I cringed, but it worked well. Weren't there any instructions with your sink?  I'm just relating the way FRANKE recommends, maybe your brand has a different method.