I didn't notice that you have spaces in your form button name that you are using for the trigger. Spaces aren't allowed in form element names in php, they are automatically converted to underscores when posted and we aren't accounting for that in the trigger. I'll get that updated in a future release.
For now, either rename the form button and re-apply the server behavior with the new trigger, or you can manually update line 38 to:
if (isset($_POST["delete_chcked_items"]) || isset($_POST["delete_chcked_items_x"])) {
Now that I tested, I also see that the checkbox value has to be set to the ID of the record that you want deleted. I've updated both of these on your page and it seems to work now.