umount.s3ql: 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/umount.s3ql - from package: s3ql

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

Error in other languages:
umount.s3ql: bevel nie gevind nie
umount.s3ql: comandă negăsită
umount.s3ql: comando não encontrado
umount.s3ql: commande introuvable
umount.s3ql: command not found
umount.s3ql: käsku ei ole
umount.s3ql: không tìm thấy lệnh
umount.s3ql: komanda nerasta
umount.s3ql: Kommando nicht gefunden.
umount.s3ql: kommandot finns inte
umount.s3ql: komut yok
umount.s3ql: nie znaleziono polecenia
umount.s3ql: níor aimsíodh an t-ordú
umount.s3ql: no se encontró la orden
umount.s3ql: no s'ha trobat l'ordre
umount.s3ql: opdracht niet gevonden
umount.s3ql: parancs nem található
umount.s3ql: perintah tidak ditemukan
umount.s3ql: príkaz nenájdený
umount.s3ql: příkaz nenalezen
umount.s3ql: команда не найдена
umount.s3ql: командата не е открита
umount.s3ql: 命令找不到
umount.s3ql: コマンドが見つかりません