I got this error many times when the JDBC URL is incorrect. Ensure that you have access to the Oracle database and the credentials are correct.
Also, if you are using the Denodo Virtual Machine to do the labs, then ensure that you should use the IP of the VM as the host name in the JDBC URI and not the localhost. From the Virtual Machine, I would execute the command "ip addr" to know the internal IP of the machine and then use it to connect to oracle data source.
For more information, refer to module Configuration Tasks of the on-demand course. You could also refer to the similar community question [here](https://community.denodo.com/answers/question/details?questionId=9060g000000Xew1AAC&title=Unable+to+establish+connection+to+Oracle+11G)
Hope this helps!