Elasticsearch Opendistro Security -- Add users to readall role

Question

I'm trying to add users to the readall role in Kibana using the elasticsearch open distro security plugin. The contents of the get request to /roles/readall is below, as is the curl syntax I'm using and the associated error. The goal here is to allow all users to read all indices regardless of tenant.

Request (removed the basic curl stuff with certs, etc):

--request PATCH -H "Content-Type: application/json" -d '[{"op": "add", "path": "/users", "value": ["*"]}]' "https://hostname/elasticsearch/_opendistro/_security/api/rolesmapping/readall"

Response:

{"status":"NOT_FOUND","message":"rolesmapping readall not found."}

Contents of the get request for the readall role:

   "readall": {
       "reserved": true,
       "hidden": false,
       "description": "Provide the minimum permissions for to readall indices",
       "cluster_permissions": ["cluster_composite_ops_ro"],
       "index_permissions": [{
           "index_patterns": ["*"],
           "fls": [],
           "masked_fields": [],
           "allowed_actions": ["read"]
       }],
       "tenant_permissions": [],
       "static": true
   }
}