chntpw: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/sbin/chntpw - from package: chntpw

You may install the required package with command apt-get install {package}

Error in other languages:
chntpw: bevel nie gevind nie
chntpw: comandă negăsită
chntpw: comando não encontrado
chntpw: commande introuvable
chntpw: command not found
chntpw: käsku ei ole
chntpw: không tìm thấy lệnh
chntpw: komanda nerasta
chntpw: Kommando nicht gefunden.
chntpw: kommandot finns inte
chntpw: komut yok
chntpw: nie znaleziono polecenia
chntpw: níor aimsíodh an t-ordú
chntpw: no se encontró la orden
chntpw: no s'ha trobat l'ordre
chntpw: opdracht niet gevonden
chntpw: parancs nem található
chntpw: perintah tidak ditemukan
chntpw: príkaz nenájdený
chntpw: příkaz nenalezen
chntpw: команда не найдена
chntpw: командата не е открита
chntpw: 命令找不到
chntpw: コマンドが見つかりません