If you come across this error when trying to perform a scheduled data refresh:
"A schedule cannot be enabled for a workbook with no external data sources."
This is because the Data Model cannot be created on the PowerPivot Server.
Your probably have two questions.
1. What is a "Data Model".
The Data Model is a cube created under the PowerPivotServer\POWERPIVOT instance:
2.Why can't/isn't the Data Model being created?
The reason why this isn't being created is because there is something wrong with the Excel Services "Data Model Settings".
You can find this be going to the Central Administration > Application Management > Manage Service Applications > Excel Services > Data Model Settings.
At this location, you need to enter the proper PowerPivotServer\Instance
Here is the proper way:
Here is a bad example (no need to have just the server name present):
Here is another bad example (PowerPivot) is misspelled:
I hope this helps.