Grading is enabled automatically when your course has at least 15 students participating in the course. To request grading for a smaller course, email Perusall Support.
If your course is connected to your institution's LMS, the gradebook is automatically enabled.
If you would like to disable automatic grading go to Settings > General, Scroll Down to "Release Scores to Students," and choose "Never (hide the gradebook and disable Perusall's automatic scoring)."