folder: 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/folder - from package: mailutils-mh
/usr/bin/mh/folder - from package: nmh

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

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