If it says that, then it should be able to access it, and it may be a bug and a good idea to contact support. See How to contact our Support Team 📧
Generally AI Studio can’t access linked files I think, unless specifically set in AI Permissions. So for files you use often it’s better to connect them directly to the rule.
Attach/link files using the paperclip icon at the bottom of the guidance window.
maybe relevant might be the permissions, which can be found in the menu below the guidance, the button with the sliders.