You cannot create a new gradebook column directly from the Gradebook in Canvas: all gradebook columns must have a corresponding assignment in Canvas. If you want to create a column in which you can record scores from manual grading, you can create a No Submission or On Paper assignment. Navigate to Assignments in the course menu and click +Assignment. Then, in the Submission Type drop-down menu, select No Submission or On Paper. For more information, visit https://community.canvaslms.com/docs/DOC-4272.