m.nviz.image: 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/lib64/grass-6.4.4/bin/m.nviz.image - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/m.nviz.image - from package: grass-core

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

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