You can easily go to WordPress Dashboard > Settings > Permalink and set the permalinks of your website on Post name. After that, the editor page will be active.
You need to have the latest version of the add-on and check the configurations of your server. Please follow this guide: https://webnus.net/dox/modern-events-calendar/what-are-best-wordpress-php-settings. If this wasn’t helpful let us know for more inspection.
Yes, you can use it with MEC Lite as well.
The add-on gets the latest information from your event and edits them. Therefore you must make sure that the last event in MEC > All Events has all the contents that the widget needs for editing.
You need to make sure that Elementor Single Builder in MEC Settings > Single Event > Single Event Style is set. For more information on how to use this add-on please follow the link below: https://webnus.net/dox/modern-events-calendar/single-builder-addon
This happens because the widgets in MEC Sidebar are not active. Make sure these widgets are active: https://webnus.net/dox/modern-events-calendar/single-events-sidebar
Let me know if you have any other questions in the comment area.