Hey Eduard,
1. What version of The Events Calendar is active on your site?
2. What version of Events Calendar Pro are you trying to activate?
This error comes from having out-of-date code, so your best option is to head to http://theeventscalendar.com/my-account/downloads and download the latest version of Events Calendar Pro which is currently version 3.11.1.
Install that on your site, if it is not already.
Then, go to wordpress.org/plugins/the-events-calendar and download the latest version of the code form there. Ensure that the The Events Calendar itself is at version 3.11.2 on your site before trying to activate Events Calendar Pro.
Let us know if this helps at all!
— George