The latest supported Modern Events Calendar plugin is 2.6.3. If you already have that version then you need to ignore the message and wait till we release the newest version. You can always check the latest supported version here.
If you actually want to update to version 2.6.3 and you have problems, please make sure that you have your theme validated.
Finally, make sure that you have a complete backup of your website, then delete the plugin and go to X > Validation and scroll down to find the plugin and install the latest version.
If you still have problems kindly get back to us with the result of the steps above and URL/User/Pass of your WordPress dashboard using the Secure Note functionality of the post to follow up the case.