Good evening Andy and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
The message you are getting is typically produced when the license key for the Events Calendar Pro is entered in the spot meant for Event Aggregator.
As long as the Events Calendar Pro add-on has been downloaded, installed and activated, it should have its own license key spot.
Could you please share a screenshot of your Events -> Settings -> Licenses screen via a private reply ?
Best regards,
Geoff B.