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