Xvfb: 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 - 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 - from package: xvfb

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

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