jellyfish: 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/lib/jellyfish/bin/jellyfish - from package: jellyfish-examples
/usr/bin/jellyfish - from package: jellyfish

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

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