If you publish one of the data sources separately as a Published Data Source, and connect the Workbook to it, that should resolve your issue.
(Either the Oracle extract with a refresh schedule, or the Excel Workbook without a schedule.)
Hope this helps
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The problem is that the Excel data source is being called the wrong way. Connections to text/Excel files should be using a UNC path e.g. \\myNetworkDrive\aFolder\maybeSomeMoreFolders\theDataSource.xlsx. You also need to make sure that the Tableau Server has access to the folder location. The "Run As User" account of the Tableau Server is what's used by the folder administrator to grant READ access. Fix this and I bet it will stop failing.
If your Excel data source is actually on the Tableau Server then do what Glen states but placing data sources on the Tableau Server is a bad idea and you shouldn't be doing that.