rdiff-backup-fs: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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

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