xvfb-run: 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 CentOS possible default full paths are:

/usr/bin/xvfb-run - from package: xorg-x11-server-Xvfb

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/xvfb-run - from package: xvfb

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

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