Jonathan is trying to use the Customise section in the GUI rather than editing the YAML for it. I’ve got a bit of both in my config and never had the issue he has so not really sure why it’s happening…
He tried both.
GUI customization only works if the entity has a unique_id (that’s what the error message says). Some integrations provide a unique_id for their entities and some don’t. Manually configured entities don’t have a unique_id by default, but for some you can add it manually in YAML.
From the docs it doesn’t look like you can configure unique_id for the file sensor, therefore it can only be custonized in YAML.