You need to be a Jira Administrator in order to manage plugins. Contact your Jira Administrator to install R4J or set up a personal Jira Cloud instance for trial purposes.
Install R4J from the Atlassian Marketplace by going to your site settings > Apps > Find new apps.
Input “R4J“ in the search box.
Select R4J from the results.
Click the Try it free button on the top right of the application page to try the application for free for 30 days.