htnotify: 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/htnotify - from package: htdig

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

On Debian possible default full paths are:

/usr/bin/htnotify - from package: htdig

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

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