inject_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/inject_alive6 - from package: thc-ipv6

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

Error in other languages:
inject_alive6: bevel nie gevind nie
inject_alive6: comandă negăsită
inject_alive6: comando não encontrado
inject_alive6: commande introuvable
inject_alive6: command not found
inject_alive6: käsku ei ole
inject_alive6: không tìm thấy lệnh
inject_alive6: komanda nerasta
inject_alive6: Kommando nicht gefunden.
inject_alive6: kommandot finns inte
inject_alive6: komut yok
inject_alive6: nie znaleziono polecenia
inject_alive6: níor aimsíodh an t-ordú
inject_alive6: no se encontró la orden
inject_alive6: no s'ha trobat l'ordre
inject_alive6: opdracht niet gevonden
inject_alive6: parancs nem található
inject_alive6: perintah tidak ditemukan
inject_alive6: príkaz nenájdený
inject_alive6: příkaz nenalezen
inject_alive6: команда не найдена
inject_alive6: командата не е открита
inject_alive6: 命令找不到
inject_alive6: コマンドが見つかりません