The actual error looked like this:
With some help, I started to look at the vRO inventory to see how the vCenter was added. Immediately an issue was seen with the vSphere vCenter plugin. Much of the information for vCenter was not populated (including the UUid). There was also an authorization issue reported for the SDK connection. This is why the workflow was failing.
To try to address the issue, we removed and re-added my vCenter. There are workflows to do just that in vRealize Orchestrator. Now when the add vCenter workflow was re-run, there was one configuration option that is critical to set correctly. This is on the second page of the wizard during the set connection properties, where it asks if you want to use a session per user method, or only a single connection to vCenter. You must select No here (it is set to Yes by default) to use only one connection to the vCenter.
If you do leave it as Yes, vCenter is not registered correctly, and workflows that rely on the vCenter UUid can fail. Once I set it to No and the wizard completed, I went back to recheck the plugin state and validate that the plugin details were now populated and that the error had gone in the inventory. Now the vCenter information (including UUid) was present.
With this resolved, the script ran fine. Kudos to Myles Gray and Hassan Hamade for their help with resolving this issue.