handbrake-gtk: 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/handbrake-gtk - from package: handbrake

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

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