You may receive the following error when running a scheduled data refresh on a PowerPivot workbook in SharePoint 2013 running PowerPivot for SharePoint:
Could not find the schedule for this work item
This error occurs when the workbook is checked out when the scheduled data refresh runs. Scheduled data refresh for workbooks that are checked out is not supported. The following article has further details:
It states that the workbook must be checked in at the time the refresh operation is finished. If the file is checked out to another user, the refreshed data will be thrown out.