Grading is enabled automatically when your course has at least 15 students participating in the course. To request grading for a smaller course, click Get help > Contact Perusall Support and submit a request.
If your course is connected to your institution's LMS, the gradebook is automatically enabled.
If you would like to disable automatic grading, navigate to Settings > General and select any option except "Automatic grading and engagement analytics." Please refer to How can I control how analytics are used in my course? to learn more about analytics options.