The PowerPivot service application identity must have Analysis Services system administrator permissions on the local SQL ...

The PowerPivot service application identity must have Analysis Services system administrator permissions on the local SQL Server Analysis Services (PowerPivot) instance. The service account must be a member of the db_owners SQL Server role for the PowerPivot service application database. It must have Full_Control on the MSAS10_50_POWERPIVOT\MSSQL\Backup folder. For more information about this rule, see http://go.microsoft.com/fwlink/?LinkID=398716