hp-levels: 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/hp-levels - from package: hplip

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

On Debian possible default full paths are:

/usr/bin/hp-levels - from package: hplip

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

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