Setting up Slack integration is possible only for the Cloud apps - free as well as the paid versions.
As a Jira admin, navigate to the ‘Manage apps’ screen. And then find the ‘Configurations’ menu in the left navigation that will be nested under ‘Automated release notes’ title. Open the ‘Integrations’ tab & it should show the action that says ‘+ Add Slack instance’. (Note that if a Slack integration is already configured, this action will not be visible.)
Click on + Add Slack instance & it will reveal a new section where this integration can be set up. This is how it will look like.
Provide the Connection name & then click ‘Connect your Slack account’. If you are already logged into your Slack workspace in that browser, you will directly see the permissions needed for the app along with the ability to Approve or Reject the integration.
Just click on ‘Allow’ & that’s it. Once the connection is successful, here’s how the integrations section for Slack would look like.
Other useful links - Troubleshooting issues in POSTing Slack messages