jack_thru: command not found

Have You tried accessing the binary with the full path?

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_thru - from package: jackd2

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