dialog: 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/dialog - from package: dialog

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

On Debian possible default full paths are:

/usr/bin/dialog - from package: dialog

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

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