How do I fix 'Error: Unable to save the resume file'?

Exit your torrent client, then open File Explorer and enter %AppData%\BitTorrent then delete the folder.  Uninstall and then reinstall your torrent client.  If this doesn’t work, create two blank files named 'settings.dat' and 'resume.dat' (without the quotes) in the same folder where you downloaded your torrent client (BitTorrent.exe).  Make sure Windows is not hiding the file extension when creating the file.