The cal and cal_list layouts require JavaScript and WordPress REST API access.
If the layouts are not displaying at all, first check that you have not disabled all WP REST API access in a security plugin. If you have, you will need to whitelist two endpoints:
/wp-json/widget-for-eventbrite-api/v1/eb_events/wp-json/widget-for-eventbrite-api/v1/wfea_cal_options
These endpoints only expose the data for the calendar, so they present no security issue as that data is going to be displayed anyway.
If the calendar layouts are still not displaying, check your browser console for errors and try to resolve them. Refer to instructions on how to debug JavaScript in your browser.
If you still have issues, raise a support ticket in the usual way.
