There is discrepancy between documentation and example section for "action.create".
The text API documentation available at: https://www.zabbix.com/documentation/3.2/manual/api/reference/action/object lists "opmessage" as "required for message operations".
However, the example section provides an API example that:
1) for alert notification, contains "opmessage" parameter
2) for recovery notification, does NOT contain "opmessage" parameter

If API call is invoked as guide by "example", the "action" gets created successfully (without "opmessage" parameter set for recovery message), BUT editing such an action via web interface will fail (see screenshot).

I believe either the example must be fixed or API calls must fail if "opmessage" is not present for "Send message" type API calls.