I followed this guide here
https://support.atlassian.com/jira-cloud-administration/docs/configure-email-notifications-for-a-custom-event/
to set up email notifications for 3 users when a certain transition occurs in their project's workflow.
The transition occurred and it's not sending these emails.

I see that there's a script runner area in there which I've left blank (says it evaluates to true when blank, which I assume means 'if this transition occurs just fire the event/email'.
I'm not sure what I'm doing wrong. Is there any other way besides using plugins to just have email notifications sent upon a transition occurring?