Because the file isn't an a executable folder, (e.g. env | grep PATH), its not able to to be executable globally.
Try adding ./ in front of it, to use the current folder.
The program 'chntpw' is currently not installed. You can install it by typing:
apt-get install chntpw
You will have to enable the component called 'universe'
chntpw version 0.99.6 100627 (vacation), (c) Petter N Hagen
chntpw: change password of a user in a NT/2k/XP/2k3/Vista/Win7 SAM file,
or invoke registry editor. Should handle both 32 and 64 bit windows
chntpw [OPTIONS] <samfile> [systemfile] [securityfile] [otherreghive] [...]
-h This message
-u <user> Username to change, Administrator is default
-l list all users in SAM file
-i Interactive. List users (as -l) then ask for username to change
-e Registry editor. Now with full write support!
-d Enter buffer debugger instead (hex editor),
-t Trace. Show hexdump of structs/segments. (deprecated debug function)
-v Be a little more verbose (for debuging)
-L Write names of changed files to /tmp/changed
-N No allocation mode. Only (old style) same length overwrites possible
See readme file on how to get to the registry files, and what they are.
Source/binary freely distributable under GPL v2 license. See README for details.
NOTE: This program is somewhat hackish! You are on your own!