parasite6: 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/parasite6 - from package: thc-ipv6

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

Error in other languages:
parasite6: bevel nie gevind nie
parasite6: comandă negăsită
parasite6: comando não encontrado
parasite6: commande introuvable
parasite6: command not found
parasite6: käsku ei ole
parasite6: không tìm thấy lệnh
parasite6: komanda nerasta
parasite6: Kommando nicht gefunden.
parasite6: kommandot finns inte
parasite6: komut yok
parasite6: nie znaleziono polecenia
parasite6: níor aimsíodh an t-ordú
parasite6: no se encontró la orden
parasite6: no s'ha trobat l'ordre
parasite6: opdracht niet gevonden
parasite6: parancs nem található
parasite6: perintah tidak ditemukan
parasite6: príkaz nenájdený
parasite6: příkaz nenalezen
parasite6: команда не найдена
parasite6: командата не е открита
parasite6: 命令找不到
parasite6: コマンドが見つかりません