You can generate the default rules list with oslopolicy-policy-generator --namespace nova. This will also give you the correct key.enter code here
If this is Newton, Nova's policies are built into the code, and no policy.json is deployed. If you are happy with the default policies, you don't need policy.json. If you need non-default policies, just create a policy.json like you did. Perhaps your rule has an incorrect key compute:create.compute:create.
You can generate the default rules list with oslopolicy-policy-generator --namespace nova. This will also give you the correct key.enter code herekey. See http://docs.openstack.org/developer/oslo.policy/usage.html.