It is likely that xsl to xls conversion is typo for something that is most likely MS Excel related conversion. Whilst some xsl file types are from database software, such as FileMaker Pro, it is unlikely that it is possible to convert xsl to xls this way.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.