Good evening Carmen and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the obstacles you have encountered while extending Event Tickets with Event Tickets Plus. I would love to help you with this topic.
I would recommend reading the following attentively: https://theeventscalendar.com/knowledgebase/new-user-primer-event-tickets/
As you can see, you will first need to actually download the Event Tickets Plus plugin and install it before you are able to enter your license key.
Once you do, you will also notice how your site’s URL is tied to your Event Tickets Plus license.
Let me know if that helps.
Have a great day!
Geoff B.