Hi @s_greathouse,
Generally, if the course is concluded, changes to grades by any method (rubric, speedgrader, direct gradebook edit) will not be allowed. Deepening how your school/institution has Canvas set up, it's possible that the course has concluded for TA-based roles, but not for teacher-based roles, in which case a teacher could make the edits. If the course is concluded for all roles, only an admin could make the adjustments. There are a few alternate workarounds for this scenario (unconclude the course, create a section with date overrides enabled and enroll the TA there, etc, but having an admin just quickly make the needed changes is probably the easiest route.
Hope this helps a bit, let us know if you have additional questions!
-Chris
Hope this helps!
-Chris