The combined network is likely the issue. This API call requires the specific network ID. This works fine for networks that are not combined but presents a problem when they are. At this time, you can contact Meraki Support to get the network ID.
There is no way that we can get this information for ourselves? If it mattered, why wouldn't it show different options for [combined network] switching and [combined network] wireless under the api/v0/organizations/<org id>/networks?
Get network IDs for use with some of the Meraki Dashboard API endpoints, such asReturn the Policy Assigned to a Client (Click-through splash page). This endpoint requires the specific wireless network ID within a combined network. Since the normal Dashboard API call to return a network only returns combined network IDs, it could be difficult to leverage this endpoint.
Network ID Formats
Combined Network (Locale) ID:L_1234567890 Single Network ID:N_1234567890
Use the Administered Orgs deep link to return the complete Org JSON.