WordPress throws this error when it tries to schedule a cron event with a name that does not exist in the list of registered intervals. The system looks for the schedule in the cron_schedules filter, fails to find it, and returns a WP_Error object.
The problem often appears after a plugin or theme that added a custom interval is deactivated, or when the cron option in the database becomes corrupted. Hosting environments that enforce fixed intervals also trigger the error.