File conversion from XLSX

File conversion from XLSX

I have attempted to upload a couple of XLSX files, allowing the conversion to the Zoho format in the process.  I get error messages.   The files are under the 10 Mb limit.  The reason for the errors is unclear.

I can upload the XLSX files without allowing the conversion, but must download them to work on them.

I've looked for a way to convert the files, once safely uploaded as XLSX files, into the Zoho format, but cannot find how to do that.  How is it done?