hp-unload: 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-unload - from package: hplip

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

On Debian possible default full paths are:

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

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

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