You need to have libreoffice installed on your server to do the conversion or use an external service like Service to convert document.
If you are using the docker image, you need to use the image suffixed with -office (i.e tryton:7.4-office) which already contains the needed dependencies.
From there its just a mather of setting the xlsxextension on your report definition. Can be done using the user interface or using the xml definition.