extlinux: command not found

Have You tried accessing the binary with the full path?

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/extlinux - from package: syslinux-extlinux

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

On Debian possible default full paths are:

/usr/bin/extlinux - from package: extlinux

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