Good afternoon Carole and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the error messages you have been getting.
I would love to help you with this topic.
If you have not already done so, I would highly recommend reading and following this article’s instructions: https://theeventscalendar.com/knowledgebase/new-user-primer-event-tickets/
One of the things you should know is that Event Tickets Plus does not replace Event Tickets, it extends it.
In other words, you need both plugins in place. Event Tickets can be installed using WordPress.org, but Event Tickets Plus needs to be manually installed (as the article explains).
Let me know how that goes.
Best regards,
Geoff B.