@Brandon_Brown
Have you tried the root account value without 'test'? Just use the production domain, even on test/beta.
edit:
While I recommend always using the production domain, even in test, this doesn't seem to be the cause of your troubles.
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The api specifically uses root_account_id, not root_account, which will be in BIGINT. The easiest way I can suggest finding this is likely the link in the Admin Tray to the Root Account, while you're in the secondary instance dashboard. The link should contain the longid. Otherwise from the root account, open the Dev tools, and see ENV.DOMAIN_ROOT_ACCOUNT_ID.
Additionally. I was unable to use the sis_user_id helper in the enrollment[user_id] param, it only took my global id, and then root_account_id became unnecessary. Not sure root_account_id if passed is even used.
I'm submitting a ticket as this affects us (obviously and others) and the endpoint should utilize the helper and root account.
So the request will not work without the global id of the user.
If you want to enroll until this is fixed CSV Import will work if you use their SIS_ID and the Root_Account name.