Currently in WFA, Snapshot Policies are not cached(in the dictionary you can see none of attributes are selected for caching), as OCUM does not discover these objects, and that's why you are seeing this "table doesn't exist" error.
In future releases, we will try to cache this if there is support from OCUM..!
But for now, to use the existing snapshot policy follow the below steps,
1) Clone the command "No-Op Cluster-Mode" and add the below piece of code in that
[parameter(Mandatory=$false, HelpMessage="Snapshot Policy Name")]
2) And, map this parameter to cm_storage.Snapshot_Policy.name in the Parameters Mapping tab and save it
3) Use this command in your workflow before the "Create CM Volume" command
4) Provide your Snapshot_Policy name in "No_Op Cluster-Mode" command details, as shown in attachment.
5) Use the above reference in your create volume command details.
For your reference I have attached the DAR file here..!
Hope this helps..!