Xdialog: 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/Xdialog - from package: xdialog

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

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