qjackctl: 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/qjackctl - from package: qjackctl

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

On Debian possible default full paths are:

/usr/bin/qjackctl - from package: qjackctl

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

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