Godot has multiple "loaders" which it goes to and ask if they are able to load a given Resource. Your error message means that none could.
This can be because :
- The Resource file, or it's loader, got corrupted.
- You are trying to open a file with an extension that Godot does not support. Like .xcf.
- A custom Resource was saved, but the file defining it is lost, or the class_name changed.