s3qlctrl: 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/s3qlctrl - from package: s3ql

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

Error in other languages:
s3qlctrl: bevel nie gevind nie
s3qlctrl: comandă negăsită
s3qlctrl: comando não encontrado
s3qlctrl: commande introuvable
s3qlctrl: command not found
s3qlctrl: käsku ei ole
s3qlctrl: không tìm thấy lệnh
s3qlctrl: komanda nerasta
s3qlctrl: Kommando nicht gefunden.
s3qlctrl: kommandot finns inte
s3qlctrl: komut yok
s3qlctrl: nie znaleziono polecenia
s3qlctrl: níor aimsíodh an t-ordú
s3qlctrl: no se encontró la orden
s3qlctrl: no s'ha trobat l'ordre
s3qlctrl: opdracht niet gevonden
s3qlctrl: parancs nem található
s3qlctrl: perintah tidak ditemukan
s3qlctrl: príkaz nenájdený
s3qlctrl: příkaz nenalezen
s3qlctrl: команда не найдена
s3qlctrl: командата не е открита
s3qlctrl: 命令找不到
s3qlctrl: コマンドが見つかりません