Hi @JacobVanVleet,
Are you a Canvas administrator? If so, you may want to run an SIS report, selecting the users and including the "deleted items" option. When you look at the report, you will probably be able to find the person with the SIS and will (in my experience) likely find out that it belongs to a deleted user. if that's the case, you'll need to reactivate the user, delete the SIS id from their account, then delete it again. After doing that, you should be able to use that SIS id again. Alternatively, you may find that the deleted user is the same person you're working on so you can just restore their account and then merge that with the other one.
Let me know if this makes sense to you, I can try to find some guides on some of this if you need to, but I have a feeling you may not need them.
-Chris
