I would open a support ticket on issues like this and work with them to troubleshoot this issue: Support Case | Tableau Software
- Provide them a screenshot of the background task admin dashboard that show the refresh failure and any error message in the hover over tooltip
- you might also need to help send logs to the support team. Work with Log Files
For issues like this, the errors are almost specific to the environment rather than how you may have set up the data connection in Tableau: The Server Run-As Account may not be able to access the directory that the Excel file is located during the refresh, the data connection may include a filepath based of a drive letter rather than the network path, the Server Run-As account cannot communicate with the database or may not have privillage to execute the custom SQL for some reason.
Either way, these would be issues to get some help to identify the warning provided by Tableau to help steer for a fix. Support would be best to help you understand if there have been any explicit notes on what needs to be fixed to resolve a Server refresh.