mu-conference: 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/sbin/mu-conference - from package: jabber-muc

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

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