nyxmms2: 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/nyxmms2 - from package: xmms2-client-nycli

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

Error in other languages:
nyxmms2: bevel nie gevind nie
nyxmms2: comandă negăsită
nyxmms2: comando não encontrado
nyxmms2: commande introuvable
nyxmms2: command not found
nyxmms2: käsku ei ole
nyxmms2: không tìm thấy lệnh
nyxmms2: komanda nerasta
nyxmms2: Kommando nicht gefunden.
nyxmms2: kommandot finns inte
nyxmms2: komut yok
nyxmms2: nie znaleziono polecenia
nyxmms2: níor aimsíodh an t-ordú
nyxmms2: no se encontró la orden
nyxmms2: no s'ha trobat l'ordre
nyxmms2: opdracht niet gevonden
nyxmms2: parancs nem található
nyxmms2: perintah tidak ditemukan
nyxmms2: príkaz nenájdený
nyxmms2: příkaz nenalezen
nyxmms2: команда не найдена
nyxmms2: командата не е открита
nyxmms2: 命令找不到
nyxmms2: コマンドが見つかりません