unflatten: 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/unflatten - from package: graphviz

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

On Debian possible default full paths are:

/usr/bin/unflatten - from package: graphviz

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

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