jack_test: 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_test - from package: jackd2

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

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