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

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