mosquitto_passwd: 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/mosquitto_passwd - from package: mosquitto

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

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