jack_capture_gui: 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_capture_gui - from package: jack-capture

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

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