In general, this error occurs when the Virtual database(i.e., denodo_monitor) used by Denodo Monitor Reports is not accessible or not created. For your scenario, I would do the following checks,
* Ensure whether the Virtual DataPort configured in the Denodo Monitor Reports is reachable.
* Check if the “denodo_monitor” Virtual database is available in the Server Explorer of the Virtual DataPort Administration Tool.
For more information, you could refer to the [Denodo Monitor Reports - User Manual](https://community.denodo.com/docs/html/document/denodoconnects/8.0/en/Denodo%20Monitor%20Reports%20-%20User%20Manual#h.f8hnk1ealjsd).
Hope this helps!