The schedule build plugin does not support scheduling a task to run at 6 o'clock each day. It only supports scheduling a job to run once, with no facility to schedule a job to run each day.
This issue report is assigned to the wrong component. Unfortunately, I don't know which component you may have been using that allows jobs to be scheduled to run at 6:00 each day. When I schedule a Freestyle project to run at 6:00 each day, it runs as expected. When I delete the Freestyle project, it no longer runs because it is deleted. Likewise for the other project types that I've used.
Please provide more details to describe the error that you are seeing. If enough details is not provided to allow others to duplicate the issue, I'll close the issue as "Cannot reproduce".