If you encounter this error:

You should read this article: https://docs.elementor.com/article/56-content-area-not-found

Dear M.E. Calendar User:

If you use the text of the shortcode:

Maybe you're having trouble

So turn on Elementor Safe Mode to be able to edit the Elementor page

Now it's enough to delete the code and place the MEC widget.

See This Video Shot: https://ufile.io/3ve3v8ap

Also any time you can deactivate MEC, and go to the page builder - remove shortcode 

and then activate MEC and go to page builder 

and finally use Modern Events Calendar (MEC) Widget  the same place previous