userpasswd: 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 CentOS possible default full paths are:

/usr/bin/userpasswd - from package: usermode-gtk

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/userpasswd - from package: usermode

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

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