dunst: 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 Debian possible default full paths are:

/usr/bin/dunst - from package: dunst

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

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