intel_backlight: 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/intel_backlight - from package: intel-gpu-tools

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

On Debian possible default full paths are:

/usr/bin/intel_backlight - from package: intel-gpu-tools

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

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