The CSD System software relies on the standard environment variable XDG_RUNTIME_DIR to be set in order to know where to create some temporary files. Some older linux systems do not set this environment variable by default, so you may need to ensure this is set yourself if you are receiving this error.
To do so, before running Mercury or Hermes run the command:
(you can use any temporary location other than /tmp that you have write permission to instead).
To have the picked up automatically, add this to your system start up scripts.