Unable to CANCEL or DELETE a Scheduled Mailing (Mosaico is installed)
If you use the Events Manger plugin try navigating to https://yoursite.com/wp-admin/edit.php?post_type=event&amp;page=events-manager-options&amp;action=update&amp;update_action=timezone-backcompat if you see a description about the timezone settings with a button to Finalise Upgrade on that page, it's likely the issue is caused by a non completed update of the Events Manager plugin.

Click on Finalise Upgrade, after the upgrade is completed you should be able to cancel/delete scheduled/unscheduled mailings.