flash_unlock: 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/sbin/flash_unlock - from package: mtd-utils

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

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