This solution is kindly provided from the Event Calendar Support:

If you're using the Classic Editor plugin after the Wordpress 5 update (Vamtam Visual composer / builder disappeared after the Wordpress 5 update), we've found a conflict with it and recurring events. We're happy to share the fact that we've now got an extension available that should help restore Recurring Events functionality in the Classic Editor.

You can find that extension for download here →

First, make a quick backup of your site and database (as you should always do before installing or updating anything on your WordPress site). Then install the extension like any other plugin.

Activate the extension, and boom! Recurring Events fields should be visible and usable again in the Classic Editor.

Please note that this is a temporary fix—we will be publishing an update of Events Calendar PRO in the coming weeks with a more robust fix baked right into it. But we didn't want to keep you waiting that long—hopefully this extension helps!