I recently ran into a situation where I was providing grades in blackboard, but didn’t require an actual submission to receive a grade, since it was just reviewing previous work. However, the assignment had a rubric that I wanted to use.
Blackboard doesn’t intend for you to do this. The point of a manual override is to go above any specific submission, and rubrics can only be used with a specific submission. Without a submissions, you can’t use a rubric.

They never intended rubrics and overrides to be used in this pattern. If you don’t have specific submissions, rubrics cannot be used.
Option 1: Set it up correctly
Assignments are different from grading columns. If you set up a grade as a Column rather than an Assignment, it will let you grade without submission.

You won’t get the “View Rubric” button if the column was set up as an assignment. This will all just be blank, because there isn’t a submission and assignments require submission to grade without an override.
Option 2: Use Excel
The workaround for this: Create an excel template with your rubric details, and use that to auto-calculate the grades. The rubric has discrete categories, so I set those up in the orange columns. Then I assign number values in the green column, and the yellow box calculates the final grade.

If you want to use and update this template, here it is.
If you have a better workaround or better googling skills, leave a comment.