Error 32 is caused by files that are currently in use when SecureClean tries to securely delete them from the system.
If SecureClean were to erase those files, it could damage either your OS or certain programs. SecureClean will want to avoid doing this, so will show the "Error 32" message notifying you that this has happened.
To avoid this, close all other programs while running the SecureClean application. If you still receive Error 32, these are files that are needed to run the OS and can not be deleted.