jack: 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/jack - from package: jack

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

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