Scalpel expects to access a single file that contains a disk image, not a mounted directory. You would first need to extract the complete disk image from the iPhone using some recovery/forensic tool.
Through iFuse, you will only see files and directories that exist according to the phone's filesystem - you can't look into the "unallocated space".
iFuse access is like accessing a Windows or NFS disk share: the system that actually contains the data controls what the clients will be able to see. In a recovery scenario, you'll need full access to the storage medium (or an exact byte-for-byte copy of it, i.e. a disk image) so that the recovery/forensic tool can examine the actual contents of what the filesystem metadata says is "unallocated space".
mountcommand to see how it's actually mounted. Explore your permission/ownership problems with github.com/waltinator/pathlld Abashscript to answer "Why can't I Read/Write that file?"..