Hi Steve,
This error occurs when the spreadsheet cannot write to the diagnostic file.
It could mean you haven’t UNZIPped the file, or you might be trying to use the spreadsheet from a cloud drive, i.e. not the hard drive on your PC.
For help unzipping the files, see this Microsoft page.
Version 1.52 has a better error message, to help you understand what might have gone wrong.
Cheers
Nick