If ABRA Flexi does not offer the option to save report exports in .xlsx format on MacOS, try changing the file type from .xls to .xlsx when saving the exported file by simply rewriting the extension.
If the file is still saved with the .xls extension, import the following XML file via Tools - Import - Import from XML:
<winstrom atomic="false">
<setting-store filter="klic eq 'XLS_EXPORT_EXTENSION'" action="delete">
</setting-store>
</winstrom>
Then log out of the currently processed company and log back in. The .xlsx format should now be available.