mixxx: 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/mixxx - from package: mixxx

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

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