mixmaster-update: 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/mixmaster-update - from package: mixmaster

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

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