ferret: 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/ferret - from package: ferret

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

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