When you try to access a device, then you need root permissions. Just use 'sudo'.
Example: sudo hfsprescue -s1 /dev/sdb2
or change to the root user with 'su'.
It's also possible that the device is mounted. Unmount the device before you run Hfsprescue. For Mac OS X see here.
© 2018 by Elmar Hanlhofer