unseen: 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/mh/unseen - from package: nmh

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

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