yad: 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/yad - from package: yad

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

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