mark: 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/mu-mh/mark - from package: mailutils-mh
/usr/bin/mh/mark - from package: nmh

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

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