whiptail: 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/whiptail - from package: newt

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

On Debian possible default full paths are:

/usr/bin/whiptail - from package: whiptail

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

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