Not sure how to fix the issue with the widget. I would try a couple different excel formats to see if that worked. Alternatively, you can create a microflow that downloads the file.
Add an action button to the list or dataview that contains the excel file. Pass the excel file as a parameter. Use the download file activity on the excel file object.
I don't think the File Document Viewer is able to display Excel files, rather, it will download them.
The error may be caused by setting Yes on the following tab for the File Document Viewer
Hope that helps,
There is an open source library you can base on to create widget to read xlsx: