isohybrid: 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/isohybrid - from package: syslinux

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

On Debian possible default full paths are:

/usr/bin/isohybrid - from package: syslinux-utils

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

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