md5: 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/tiger/bin/md5 - from package: tiger

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

Error in other languages:
md5: bevel nie gevind nie
md5: comandă negăsită
md5: comando não encontrado
md5: commande introuvable
md5: command not found
md5: käsku ei ole
md5: không tìm thấy lệnh
md5: komanda nerasta
md5: Kommando nicht gefunden.
md5: kommandot finns inte
md5: komut yok
md5: nie znaleziono polecenia
md5: níor aimsíodh an t-ordú
md5: no se encontró la orden
md5: no s'ha trobat l'ordre
md5: opdracht niet gevonden
md5: parancs nem található
md5: perintah tidak ditemukan
md5: príkaz nenájdený
md5: příkaz nenalezen
md5: команда не найдена
md5: командата не е открита
md5: 命令找不到
md5: コマンドが見つかりません