alive6: 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/alive6 - from package: thc-ipv6

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

Error in other languages:
alive6: bevel nie gevind nie
alive6: comandă negăsită
alive6: comando não encontrado
alive6: commande introuvable
alive6: command not found
alive6: käsku ei ole
alive6: không tìm thấy lệnh
alive6: komanda nerasta
alive6: Kommando nicht gefunden.
alive6: kommandot finns inte
alive6: komut yok
alive6: nie znaleziono polecenia
alive6: níor aimsíodh an t-ordú
alive6: no se encontró la orden
alive6: no s'ha trobat l'ordre
alive6: opdracht niet gevonden
alive6: parancs nem található
alive6: perintah tidak ditemukan
alive6: príkaz nenájdený
alive6: příkaz nenalezen
alive6: команда не найдена
alive6: командата не е открита
alive6: 命令找不到
alive6: コマンドが見つかりません