There is a timer job called “PowerPivot Management Dashboard Processing Timer Job” that, by default, runs daily. This process relies on the secure store service to authenticate to the data source. If the permissions on the SSS target application are not correct, you will receive an “Access is denied to the secure store service” error when this timer job runs.
Check the SSS ID for the Refresh Account
$ppv = Get-PowerPivotServiceApplication
$ppv | fl *Account*
Next, open the SSS and edit the Target Application that corresponds to the one that the PowerPivot service application is using.
On the third page of the edit wizard, ensure that the farm account and the service application account for PowerPivot are in the Members list:
Lastly, Re-Run the Dashboard Processing Timer Job by clicking Run Now on the timer job definition page, or you can run script below.
Get-SPTimerJob “PowerPivot Dashboard Processing Timer Job” | Start-SPTimerJob
You should now see that the cube has been processed on the PowerPivot management dashboard!