twopi: 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/twopi - from package: graphviz

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

On Debian possible default full paths are:

/usr/bin/twopi - from package: graphviz

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

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