unix_update: 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_update - from package: pam

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

On Debian possible default full paths are:

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

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

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