Hi Emmaleigh,
This might be a better question for your local Canvas Admins. The default TA permissions in Canvas do allow for grading. So if you have added your TA to the course with TA permissions and they cannot grade it probably means your local Canvas Admins have changed the permissions.
If you need to know how to add the TA to the course here is a quick guide https://community.canvaslms.com/docs/DOC-12973-4152724200 Make sure to set the role to TA.
Rick
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