You may receive the following error when running a scheduled data refresh on a PowerPivot workbook in SharePoint 2013 running PowerPivot for SharePoint using SQL PowerPivot 2012 SP1:
A schedule cannot be enabled for a workbook with no external data sources.
This can happen when a PowerPivot workbook created in Excel 2010 using the PowerPivot 2012 add-in is used in SharePoint 2013, and the Excel Services Authentication Setting in the data connection properties is set to anything other than Windows Authentication:
If you upload the workbook to SharePoint 2013 and you go to the workbook's Manage Data Refresh page from the PowerPivot Gallery, you will receive the error. This is because the SSS and None options cannot be used in this scenario. Change the setting to Windows Authentication and save the workbook back to SharePoint to fix the error and enable the scheduled data refresh for the workbook.