Flatpack distributions are not official distributions, and not supported via this site.
You can find the official distributions via https://freefilesync.org/download.php
That command as show includes a space in the path for your ffs_batch file. The space is the first character in the path, and would make the path invalid.
Once you get past past that problem, sometimes flatpak might need special permission for accessing your home directory or contents beneath it. You might want to check what permissions the flatpak has, and you might need to give it exact permissions.
