hp-scan: 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-scan - from package: hplip

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

On Debian possible default full paths are:

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

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

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