Anything else? That error is already fixed in an unreleased version. I don’t think it should be stopping the taxonomy from being created though. Speaking of, do you see Priority in the WP backend?
The problem is the theme uses a custom post type for tickets, as opposed to the default “posts” post type. When someone posts via email, there is no way the theme can specify to use the “ticket” post type. (That I know of).
Those names are taken from the labels defined when the custom taxonomy is registered. To change this, you can filter the labels when the taxonomy is registered in your child theme: