That would suggest that your JIRA system does not have the right SSL certificates installed to be able to reach the system being called in the webhook.
You'll need to get the client certificates and install them in the JIRA JVM.
Here is answer for you: https://confluence.atlassian.com/kb/unable-to-connect-to-ssl-services-due-to-pkix-path-building-failed-779355358.html
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