The Java action to export Excel files requires a parameter of type System.FileDocument or a specialization of that entity. The typical configuration is that you create a context object for the export, make an association between that context object and the List of objects you want in your spreadsheet and then in the template configure a sheet with the context object and the list you want in the file.
The Java action will fail if you don’t pass it a file document as one of the parameters because it needs to have a file to place the generated Excel content into.