6to4test.sh: 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/6to4test.sh - from package: thc-ipv6

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

Error in other languages:
6to4test.sh: bevel nie gevind nie
6to4test.sh: comandă negăsită
6to4test.sh: comando não encontrado
6to4test.sh: commande introuvable
6to4test.sh: command not found
6to4test.sh: käsku ei ole
6to4test.sh: không tìm thấy lệnh
6to4test.sh: komanda nerasta
6to4test.sh: Kommando nicht gefunden.
6to4test.sh: kommandot finns inte
6to4test.sh: komut yok
6to4test.sh: nie znaleziono polecenia
6to4test.sh: níor aimsíodh an t-ordú
6to4test.sh: no se encontró la orden
6to4test.sh: no s'ha trobat l'ordre
6to4test.sh: opdracht niet gevonden
6to4test.sh: parancs nem található
6to4test.sh: perintah tidak ditemukan
6to4test.sh: príkaz nenájdený
6to4test.sh: příkaz nenalezen
6to4test.sh: команда не найдена
6to4test.sh: командата не е открита
6to4test.sh: 命令找不到
6to4test.sh: コマンドが見つかりません