mllp_send: 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/mllp_send - from package: python-hl7

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

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