unix_chkpwd: 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:

/sbin/unix_chkpwd - from package: pam

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

On Debian possible default full paths are:

/sbin/unix_chkpwd - from package: libpam-modules-bin

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

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