smurf6: 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/smurf6 - from package: thc-ipv6

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

Error in other languages:
smurf6: bevel nie gevind nie
smurf6: comandă negăsită
smurf6: comando não encontrado
smurf6: commande introuvable
smurf6: command not found
smurf6: käsku ei ole
smurf6: không tìm thấy lệnh
smurf6: komanda nerasta
smurf6: Kommando nicht gefunden.
smurf6: kommandot finns inte
smurf6: komut yok
smurf6: nie znaleziono polecenia
smurf6: níor aimsíodh an t-ordú
smurf6: no se encontró la orden
smurf6: no s'ha trobat l'ordre
smurf6: opdracht niet gevonden
smurf6: parancs nem található
smurf6: perintah tidak ditemukan
smurf6: príkaz nenájdený
smurf6: příkaz nenalezen
smurf6: команда не найдена
smurf6: командата не е открита
smurf6: 命令找不到
smurf6: コマンドが見つかりません