nop: 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/nop - from package: graphviz

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

On Debian possible default full paths are:

/usr/bin/nop - from package: graphviz

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

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