jack-keyboard: 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-keyboard - from package: jack-keyboard

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

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