The system-files interface is used to access files from the host file-system, outside of the user’s home directory, which are not normally exposed to snaps. As such when using this interface, a snap needs to explicitly list which files / paths it requires access to.
However, currently pyflow simply plugs system-files but does not list access to any particular files. You need to list which files / paths are required and what access is required to these (read/write) as per the documentation when plugging this interface - so please update your snapcraft.yaml with specific paths, and then mention in this request post why access to those paths is required so that the review can take place.