This error happens because the Java environment does not have information about the HTTPS server to verify that it is a valid website. It is a common error and the workaround for it will be:
Go to Manage Plugins > Plugin manager > Advanced
You should see the "Update site" option which happens to have the link with https protocol. Just remove the 's' after 'http' in the URL provided and submit the changes and click on "Check Now".
You should now be able to send your notifications.
Hope this helps.
For more information, check out this jenkins tutorial.