mix: 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/lib/phylip/bin/mix - from package: phylip

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

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