xfpm-power-backlight-helper: 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/sbin/xfpm-power-backlight-helper - from package: xfce4-power-manager

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

On Debian possible default full paths are:

/usr/sbin/xfpm-power-backlight-helper - from package: xfce4-power-manager

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

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