Hi Ian,
Thanks for reaching out, and I’m so sorry for the trouble here! This error message is typically displayed when a license key for Events Calendar PRO or another plugin is entered into the license key field for Event Aggregator. Please double-check that you’ve installed and activated both The Events Calendar and Events Calendar PRO on your site, and ensure that your key for Events Calendar PRO has been entered into the appropriate field!
If you’re still experiencing issues after confirming that all of the above looks okay, would you mind dropping us a quick email with a reference to this thread? We’ll be glad to generate a new license for your account to see if this resolves your problem. (Please select the option “Help accessing my account” from the drop-down menu to display the full email form.)
https://theeventscalendar.com/contact/
Thanks, and let me know if you should have any other questions or concerns here!