Boards are specifically a feature of Jira Software. While you might have upgraded Jira Core here, it doesn't necessarily mean that you also updated the Jira Software plugin here. This will need to happen in order to see that Boards tab in the navigation bar.
In Jira 7 and higher versions, you should be able to quickly install this in your existing Jira site by going to Settings -> Applications -> Versions and Licenses. On this page you can see your Jira product licenses, and provided you have a valid license for Jira Software, this page should let you install the compatible version of JSW with Jira Core directly. If this is installed, and it's still not working, then your plugin likely failed to load on startup for some reason.
If that happens, I would recommend taking a closer look at your $JIRAHOME/log/atlassian-jira.log file immediately after startup. It should tell you more about what plugins might have failed to start up. One common cause for this behavior is that upgrading Jira tends to overwrite your $JIRAINSTALL/bin/setenv.sh or .bat file. This file is one way that Jira can use to set JVM parameters such as how much memory Jira should be using in the Xmx parameter. The default is 768m. That works fine for fresh installs, but it tends to be woefully inadequate for larger Jira sites. If that is the case here, you might want to look at the steps for Increasing Jira memory and Jira sizing guide to get an idea of what kind of resources you need to run Jira well..
If you already heard about Smart Commits in Bitbucket, know that you just stumbled upon something even better (and smarter!): Genius Commits by Better DevOps Automation for Jira Data Center (+ Server...
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