Good evening Patrick and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the issues you have encountered while trying to activate Event Tickets Plus on your website. I would love to help you with this topic.
What I believe is going on is that you need to first download and install the Event Tickets Plus premium add-on before being able to register its license key on your website.
Please walk-through the following procedure: https://theeventscalendar.com/knowledgebase/new-user-primer-event-tickets/
Let me know how that goes.
Best regards,
Geoff B.