I copied the webhook token back into the URL in the Jira webhook setup, so it says https://example.com/jira/issues/webhook?t=abcdefghijk, and also your discourse_jira_url is supposed to be https://modernsquared.atlassian.net/ without the path.
@vinothkannans I have another question though: Is it possible to change which issue is attached to a topic after attaching, or simply remove/delete the attachment? If not, have you thought about implementing it?