The mentioned error message is generally thrown when:
* The JDBC URI is incorrectly defined in the Data Source Configuration of the Virtual Dataport Administration Tool. It could be either incorrect port number or incorrect hostname or missing “@” symbol in the URI.
* Oracle server listener service is down thus not responding to requests from clients on the designated port number.
Hence, I would start with validating the JDBC URI syntax which should follow the below format:
* jdbc:oracle:thin:@<host>:<port>:<service name> or
If the JDBC syntax looks right, I would then verify if the Oracle server listener services are up and running.
Hope this helps!