archfs: 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/archfs - from package: rdiff-backup-fs

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

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