Hi,
Sorry for the issues you are having. I can help troubleshoot this with you.
First that error message in the admin is displaying because your theme has template files for WooCommerce in it and they are from an older version.
Second, I visited your site and was able to add tickets to the cart without issue.
I noticed your link for the cart has ?wootickets_process=1.
We removed that for 3.9 so maybe you were running off a cached copy of the site. Can you clear your browser cache or try another and see if that helps?
Let me know and we can go from here.
Thanks