qvfb: 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/lib/x86_64-linux-gnu/qt4/bin/qvfb - from package: qt4-dev-tools

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

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