The possible causes of this problem are:
The scan driver is not installed in the /usr/lib/sane directories.
The scan driver device file does not exist.
The scan driver is not listed in the /etc/sane.d/dll.conf file.
Make sure your SANE installation is using the directory structure /usr/lib/sane and /etc/sane.d/.
For some SANE installations, you may need to set the LD_LIBRARY_PATH. In the command line, type
.Verify that 'mfp_nscan' is listed in the /etc/sane.d/dll.conf file and not commented out.