mcxi: 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/mcxi - from package: mcl

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

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