When I tried to connect to Oracle DB in the Denodo Quickstart Virtual Machine, I was able to connect successfully. Some of the general reasons for this error could be,
* The **version** of **DQVM**, please check you are using the latest version of DQVM which is,
* DQVM version (20200128) for VirtualBox (1.5 GB).
* DQVM version (20200128) for VMware (1.6 GB).
* DQVM version (20200206) for Hyper-V (1.5 GB).
* From the error, it looks like the Oracle listener is running but not the database service. I would suggest you to check if you have allocated the recommended 4GB memory for the Virtual machine.
You could download the latest version of the Virtual Machine from the Configuration section of your On-demand course. For more information on the recommended Hardware requirements, refer to the **Denodo Training QuickStart Virtual Machine** User Manual.
Hope this helps!!