upower: 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 Debian possible default full paths are:

/usr/bin/upower - from package: upower

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

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