Sometimes a functioning Excel installation is not recognised and you get a warning that spreadsheets cannot be handled. Try to do a repair install of excel, even if it looks perfectly fine by itself. Unfortunately this is something that I have no control over; it seems to be related to the way that installers handle some excel components, especially when updating Office.
If that does not help, you can install LibreOffice or OpenOffice, both support all formats that Excel does and are (hopefully) picked up by the converter if Excel is not. You may have to enforce the use of Libre/Open Office, see the section about command line use. That section also describes how to get a debug log in case of Excel issues. If you have problems with Excel, please send that debug log.
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