ldns-notify: 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/ldns-notify - from package: ldns

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

On Debian possible default full paths are:

/usr/bin/ldns-notify - from package: ldnsutils

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

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