You're using an Excel file that contains a Excel Power Pivot Model with external data connections and currently the data refresh in Excel Online is working without any issues for the internal users of the organization tenant.
When this file is shared with external users of the tenant, this users are not able to refresh the file from the Excel Online. They receive an error message like:
"An error occurred while working on the data model in the workbook. Try again.
One or more data connections in this workbook can not be refreshed."
"An error occurred while working with the data model in the book. Try again.
Failed to update the data connections in this book.
The failure occurred when the following connections were updated:"
Currently, the data refresh operation in Excel Online doesn’t support external users of the tenant.
As workaround, the user can possibly open the file into the Excel client application and refresh the data source from there.