It also says to go look into the logs.
If you have the access, ssh to the machine and tail the '<jira_home>/log/atlassian-jira.log' while trying to enable it. Then you should see the actual problem and we can go from that stacktrace.
Although, Structure is no small plugin and has been around for a long time and many different instances, so this is unlikely to happen. Did you download any specific version from the marketplace - can you check that the version you are trying to install is actually supported by your Jira instance, and that it is the correct Server or Data Center deployment version?
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