zenity: 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/zenity - from package: zenity

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

On Debian possible default full paths are:

/usr/bin/zenity - from package: zenity

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

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