You'll have to grant the
Createruntime permissions on the Entity Services Resource. Go to System>Resources and on the Right side of the entity on the Resources list, select the permissions icon and go to runtime. Under Services Overrides, Select Create Thing and add the System user to the service permissions.
The System account already had "Service Execute" on the EntityServices resource for all properties, events and services. I also explicitly added the permissions in an override for CreateThing and I get the same error. I've also tried explicitly giving the user account permissions to the CreateThing service and that hasn't worked either