simple-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 Debian possible default full paths are:

/usr/bin/simple-scan - from package: simple-scan

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

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