This is a common problem encountered when the torrent client is attempting to write to a file that is presently being accessed by another process or application. Close all applications that could possibly open such a file, including Finder and Explorer windows. If this continues to occur, restart the torrent client. Restarting your system may be required as well.
When downloading or seeding, I get 'Error: The system cannot find the path specified'.
This problem is often caused by a limitation in Windows with path lengths. Unfortunately, Windows limits the maximum length to 255 characters including the filename. Try saving the torrent to a location closer to the drive's root, such as C:\torrent.