Sorry, which sidebar are you referring to? If this is the left side project navigation bar, then you can collapse this by clicking the box at the bottom of the screen:
If this isn't the sidebar you are referring to perhaps you can grab a screenshot to let us know which specific page/area you are asking about.
There is an open feature request to allow this to be hidden completely from users in https://jira.atlassian.com/browse/JRASERVER-62709
And while users can click that button I highlighted in red in order to collapse that pane, Jira users that can login to this page are expected to be able to see this project navigation view and in turn they can choose to either show this sidebar or collapse it.
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