thcsyn6: 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/thcsyn6 - from package: thc-ipv6

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

Error in other languages:
thcsyn6: bevel nie gevind nie
thcsyn6: comandă negăsită
thcsyn6: comando não encontrado
thcsyn6: commande introuvable
thcsyn6: command not found
thcsyn6: käsku ei ole
thcsyn6: không tìm thấy lệnh
thcsyn6: komanda nerasta
thcsyn6: Kommando nicht gefunden.
thcsyn6: kommandot finns inte
thcsyn6: komut yok
thcsyn6: nie znaleziono polecenia
thcsyn6: níor aimsíodh an t-ordú
thcsyn6: no se encontró la orden
thcsyn6: no s'ha trobat l'ordre
thcsyn6: opdracht niet gevonden
thcsyn6: parancs nem található
thcsyn6: perintah tidak ditemukan
thcsyn6: príkaz nenájdený
thcsyn6: příkaz nenalezen
thcsyn6: команда не найдена
thcsyn6: командата не е открита
thcsyn6: 命令找不到
thcsyn6: コマンドが見つかりません