x11vnc: 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/x11vnc - from package: x11vnc

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

On Debian possible default full paths are:

/usr/bin/x11vnc - from package: x11vnc

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

Error in other languages:
x11vnc: bevel nie gevind nie
x11vnc: comandă negăsită
x11vnc: comando não encontrado
x11vnc: commande introuvable
x11vnc: command not found
x11vnc: käsku ei ole
x11vnc: không tìm thấy lệnh
x11vnc: komanda nerasta
x11vnc: Kommando nicht gefunden.
x11vnc: kommandot finns inte
x11vnc: komut yok
x11vnc: nie znaleziono polecenia
x11vnc: níor aimsíodh an t-ordú
x11vnc: no se encontró la orden
x11vnc: no s'ha trobat l'ordre
x11vnc: opdracht niet gevonden
x11vnc: parancs nem található
x11vnc: perintah tidak ditemukan
x11vnc: príkaz nenájdený
x11vnc: příkaz nenalezen
x11vnc: команда не найдена
x11vnc: командата не е открита
x11vnc: 命令找不到
x11vnc: コマンドが見つかりません