Hey Robert,
I’m sorry you’ve run into this error. This will happen if the wp_mail() email function within WordPress returns “false”, which itself can happen for a number of reasons.
To begin troubleshooting this, can you first try seeing if any errors pop up on your site if you head to your site’s wp-config.php file and change this line of code:
define('WP_DEBUG', false);
to this:
define('WP_DEBUG', true);
Once you make that change, try emailing the Attendees List again and see if any PHP errors display – if they do, copy and paste them in their entirety into a Gist at http://gist.github.com, then share a link to that Gist here.
In the meantime, can you also clarify each of the following details?
1. What version of The Events Calendar is active on your site?
2. What version of WooCommerce Tickets is active on your site?
3. What version of WordPress itself is your site running?
Thank you!
George