I've answered so many questions about the Sculpt Tools UI not working properly, especially Grease Cut, and I just don't get it. I'm not experiencing the same issues that keep getting reported.
See here I'm using 2.79 in perspective view, noting grease pencil is set to "Object" mode instead of Scene:
Granted the addon has *always* been finicky with only a 50-60% success rate for grease cut. But do it enough times and it always works for me eventually. I know that ideally it would work perfectly, but hey, it's a free tool that isn't developed anymore. Either don't use the tool or be appreciative that it works at all.