I recently worked with a customer who was clicking on a slicer in an Excel Online PowerPivot workbook and getting the below error:
After many days of analyzing logs, it was determined the cube was showing as being "Unprocessed". Therefore the refresh could not complete. The fix was to re-calculate the workbook (in PowerPivot).
To do this, you will need open the workbook and go into PowerPivot > Manage > Design > Calculations Options.
You may need to flip it to Manual Calculation Mode which will then allow you to choose Calculate Now.
After you Manually (re)Calculate, you will want to re-select Automatic Calculation Mode, Re-Save to Sharepoint and test.
I hope that helps.