Enable Dark Feature in Jira
Some of the features in JIRA are not enabled by default (there could be many reasons for that) and implemented as a Dark Features.
You should be very careful with those features and should change it when you explicitly asked by Atlassian support.
To enable/disable Dark Features you need to be JIRA admin. First, go to the page: <Jira_URL>/secure/SiteDarkFeatures!default.jspa .
In order to enable a feature flag, type <feature_flag_name>.enabled in the "Enable dark feature" textbox and click the "Add" button. Make sure to not have double ".enabled" suffix at the end of feature flag name.
To disable a feature flag, either click "Disable" link next to a feature flag in the "Enabled dark features" section (as on a screenshot below) or type <feature_flag_name>.disabled feature flag in the "Enable dark feature" textbox and click the "Add" button. Make sure to not have the combination of suffixes - ".enabled.disabled" - at the end of the feature flag name.