Adjust your proxy or firewall settings
https://confluence.atlassian.com/jirakb/how-to-configure-an-outbound-http-and-https-proxy-for-jira-applications-247857187.html
Step 4 : Setup permissions , Click on OAuth & Permissions and setup permission as below
Add slack team done
To test Slack notification : Create an issue , then you will got a message in configured channel
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Email: jsoftconnector@gmail.com, robert@jsoftware.io