If you would like to use a rubric for grading in SpeedGrader on a Turnitin assignment, you will first have to create/add the rubric to the assignment before enabling Turnitin.
If you have already enabled Turnitin on an assignment, you can temporarily remove Turnitin and follow these steps.
Also, note that for students to see the rubric that you have attached, they will have to go to their Grades Details page and then click the Show Rubric button in the upper right-hand corner. If you would like for students to also be able to view a rubric directly on the assignment, you would have to create a rubric in the Turnitin settings as well.
If you have already enabled Turnitin on an assignment and would like to add a rubric or change/edit a rubric that was already on the assignment, you can still do so.
If you temporarily make these changes to add the rubric, once the External Tool has been re-added to the assignment, the Turnitin LTI will re-link any submissions students had already submitted to the assignment. Students’ submissions, scores, or grades will not be lost.
Note: You also have the option of using the built-in Turnitin plagiarism checker when creating or editing an assignment. This option can be used in conjunction with a Canvas rubric whether or not Turnitin has already been enabled for the assignment.