Howdy Caldia,
I see you don’t have a multisite license. I am sorry upgrading is a pain right now for multisites without the full license, but we are definitely working to fix that with an updated version of the plugin.
In the mean time the best way to upgrade would be to delete the old plugin and install the new. You don’t even need to use a file manager if you don’t want, simply go to WP Admin > Plugins, hit deactivate, then delete. Now install the latest version. The reason you are having that fatal error is because you haven’t delete the old one, so the two running side by side are conflicting. Could you try deleting theĀ events-calendar-pro.old folder? That should cause the error to go away. Does it?
Cheers!
– Brook