jack_unload: 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 CentOS possible default full paths are:

/usr/bin/jack_unload - from package: jack-audio-connection-kit

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/jack_unload - from package: jackd1
/usr/bin/jack_unload - from package: jackd2

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

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