Good evening Jeanne and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the orders not auto completing in spite of the use of the checkbox.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Full disclosure, on certain installations, that option does not work properly for unknown reasons.
You can try adding the following snippet in the functions.php of your WordPress theme to see if that works: https://docs.woocommerce.com/document/automatically-complete-orders/
Let me know how that goes.
Best regards,
Geoff B.