This is often because the Jira instance is not publicly accessible. Make sure your instance is routable (i.e. our server can reach it!). If not, you'll need to put your Jira instance on the internet so that we can reach it.
Unfortunately, this can vary per user. This error means we couldn't connect to your server from our server. For example, perhaps your proxy is not configured correctly, or not set to allow our IP. If you need to whitelist our IPs, you can find a list of our IP ranges here.