You have configured dbconfig.xml for JIRA 7.x to use Oracle 12. But JIRA is not working correctly when you try to start it. What should you do with your dbconfig.xml file?
Atlassian's website, as of 8/30/18, recommends having "oracle10g", an unsupported version, in the dbconfig.xml file. This is despite the fact that Oracle 10 and Oracle 11 are not supported for JIRA 7.x according to Atlassian's site here.
The XML tag for the database type in dbconfig.xml (a file for JIRA) should specify version 10 -- even though it is not supported! Atlassian's website shows the correct tag in the dbconfig.xml file on their website as this:
The tag should not say "oracle11g" or "oracle12g". It should say a version that is not supporte: "oracle10g".