ferret_c: 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_c - from package: ferret-vis

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

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