Right now, if there is a rubric attached to an assignment, the grades can only be entered through the rubric. You should be able to go into the gradebook and edit and enter grades by hand.
Hi there, I'm sure that your submission was just before we had this in place, but if you go to the gradebook and view the rubric for a student there is a link at the top for adjusting the grade. This will open options for you to add points, remove points, or mark score as zero. Please don't hesitate to get in touch if you have additional feedback.
I would love to be able to override the rubric grade which is similar to this idea. We have teachers who need to deduct points for students who are late with their assignment.