So an update.
I removed the vcns checkbox and still get the error. I think my vCAC server can't communicate with vCO but I don't know why.
I rebuilt both vCO and vCAC and made sure they were all on the same SSO Server but that didn't make any difference. Also a complete rebuild of both vCAC and vCO left me with the same error.
Is anyone successfully using vCO 5.5.3 and vCAC 6.1 together?
Workflow 'vSphereVCNSInventory' failed with the following exception: Could not reach vCenter Orchestrator endpoint {0}. Trying the next highest priority endpoint.