xbacklight: 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/xbacklight - from package: xbacklight

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

On Debian possible default full paths are:

/usr/bin/xbacklight - from package: xbacklight

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

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