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

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

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