Are you using SystemRescueCD...
chntpw is not in fact a command, it is a program with commands. Can you invoke it with the "-h" option to see that it is there and working?
You will not see anything happen if you are not in the correct directory. The file address given in the screen shot as an example may not be correct. I saw it elsewhere as
You might google or use a live-cd to find and check on what the file name is in your Windows. All the variants I have seen end with"/system32/config" or "/System32/config".