qstail: 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/qstail - from package: libapache2-mod-qos

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

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