mlpack_nmf: 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/mlpack_nmf - from package: mlpack-bin

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

On Debian possible default full paths are:

/usr/bin/mlpack_nmf - from package: mlpack-bin

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

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