covert_send6d: 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/covert_send6d - from package: thc-ipv6

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

Error in other languages:
covert_send6d: bevel nie gevind nie
covert_send6d: comandă negăsită
covert_send6d: comando não encontrado
covert_send6d: commande introuvable
covert_send6d: command not found
covert_send6d: käsku ei ole
covert_send6d: không tìm thấy lệnh
covert_send6d: komanda nerasta
covert_send6d: Kommando nicht gefunden.
covert_send6d: kommandot finns inte
covert_send6d: komut yok
covert_send6d: nie znaleziono polecenia
covert_send6d: níor aimsíodh an t-ordú
covert_send6d: no se encontró la orden
covert_send6d: no s'ha trobat l'ordre
covert_send6d: opdracht niet gevonden
covert_send6d: parancs nem található
covert_send6d: perintah tidak ditemukan
covert_send6d: príkaz nenájdený
covert_send6d: příkaz nenalezen
covert_send6d: команда не найдена
covert_send6d: командата не е открита
covert_send6d: 命令找不到
covert_send6d: コマンドが見つかりません