This error can be given when the scheduled task was not run because it was already running. The scheduled task has the setting "Do not start a new instance" selected for the setting "If the task is already running, then the following rule applies" (which is the default when creating scheduled tasks in SyncBack). You can verify this by looking at the History of the scheduled task in the Task Scheduler. There will be an event "Launch request ignored, instance already running".

If that is not the cause, try deleting and re-creating the scheduled task from within SyncBack.